The Banqiao dam was built in the early 1950s on the Ru River as part of a project to control flooding and generate electricity and as a response to severe flooding in the Huai River Basin in 1949 and 1950 The dam was 118 meters high (387 ft) and had a storage capacity of 492 million m³ (398,000 acre feet), with 375 million m³ (304,000 acre feet) reserved for flood storage. Cracks in the dam and sluice gates appeared after completion due to construction and engineering errors. They were repaired with advice from Soviet engineers and the new design, dubbed the iron dam, was considered unbreakable.
Chen Xing was one of China's foremost hydrologists and was involved in the design of the dam. He was also a vocal critic of the government dam building policy, which involved many dams in the basin. He had recommended 12 sluice gates for the Banqiao Dam, but this was scaled back to 5 and Chen Xing was criticized as being too conservative. Other dams in the project, including the Shimantan Dam, had similar reduction of safety features and Chen was removed from the project. In 1961, after problems with the water system surfaced, he was brought back to help. Chen continued to be an outspoken critic of the system and was again removed from the project.
The Dam was designed to survive a 1-in-1,000-year flood (of rainfall per day). In August of 1975, however, a 1-in-2,000 year flood occurred, poured more than a year's rainfall in 24 hours (new records were set, at rainfall per hour and per day, exceeding the average annual precipitation of about ), which weather forecasts failed to predict, produced by the collision of Super Typhoon Nina and a cold front. Communication to the dam was largely lost due to the collapse of buildings under heavy rain and wire failures. On August 6, a request to open the dam was rejected, because of the existing flood in downstream areas. On August 7, however, the request was accepted, but the telegraphs failed to reach the dam
The sluice gates were not able to handle the overflow of water, partially due to sedimentation blockage. On August 7 at 7:30 p.m., the People's Liberation Army Unit 34450, which was deployed on the Banqiao Dam, sent the first dam failure warning via telegraph. On August 8, 12:30 a.m., the smaller Shimantan Dam, which was designed to survive a 1-in-500-year flood, failed to handle more than twice its capacity and broke upstream, only 10 minutes after Unit 34450 sent a request that would open the Banqiao Dam by air strike. A half hour later, at 1:00 a.m., water crested at the Banqiao Dam and it too failed. This precipitated the failure of 62 dams in total. The runoff of Banqiao Dam was per second in vs. per second out, and 701 million tons of water was released in 6 hours, while 1,670 million tons of water was released in 5.5 hours at upriver Shimantan Dam, and 15.738 billion tons of water was released in total.
The resulting flood waters caused a large wave, which was wide, high in Suiping (遂平), to rush downwards into the plains below at nearly , almost wiped out an area long, wide, and created temporary lakes as large as . Seven county seats, namely Suiping, Xiping(西平), Ru'nan (汝南), Pingyu (平舆), Xincai (新蔡), Luohe (漯河), Linquan (临泉), were inundated, as were thousands of square kilometers of countryside and countless communities. Evacuation orders had not been fully delivered because of weather conditions and poor communications. Telegraphs failed, signal flares fired by Unit 34450 were misunderstood, telephones were rare, and some messengers were caught by the flood. While only 827 out of 6,000 people died in the evacuated community of Shahedian just below Banqiao Dam, half of a total of 36,000 people died in the unevacuated Wencheng commune of Suipin County next to Shahedian, and the Daowencheng Commune was wiped from the map, killing all 9,600 citizens. Although a large number of people were reported lost at first, many of them returned home later. Tens of thousands of them were carried by the water to downriver provinces and many others fled from their homes.
To protect other dams from failure, several flood diversion areas were evacuated and inundated, and several dams deliberately destroyed by air strikes to release water in desired directions. The Nihewa and Laowangpo flood diversion areas downstream of the dams soon ran out of their capacity and gave up part of their storage on August 8, forcing more flood diversion area to begin to evacuate. The dikes on the Quan River collapsed in the evening of August 9, and the entire Linquan county in Fuyang, Anhui was inundated. As the Boshan Dam, with a capacity of , crested, and the water released from the failures of Banqiao and Shimantan was rushing downstream, air strikes were made against several other dams to protect the Suya Lake dam, which was already holding 1.2 billion m³ of water Suya Lake only won a temporary reprieve, and both it and Boshan became targets as well. Finally, the Bantai Dam, which was holding 5.7 billion m³ of water, was bombed.
The Jingguang Railway, a major artery from Beijing to Guangzhou, was cut off for 18 days, as were other crucial communications lines. Although 42,618 People's Liberation Army troops were deployed for disaster relief, all communication to and from the cities was cut off. Nine days later there were still over a million people trapped by the waters, relying on airdrops of food and unreachable to disaster relief. Epidemics and famine devastated the trapped survivors. The damage of the Zhumadian area was estimated 3.5 billion Renminbi yuan The Zhumadian government appealed to the whole nation for help, and received more than 300 millions Renminbi in donations .
After the flood, a summit of National Flood Prevention and Reservoir Security at Zhengzhou, Henan was held by the Department of Water Conservancy and Electricity, and a nationwide reservoir security examination was performed after this meeting. Chen Xing was again brought back to the project and aided in clearing the river channels.
The death toll of this disaster was declassified in 2005.