Abstract: In the summer of 1998, an exceptionally serious flood, with the characteristics of high water level, large volume of flow, long duration and serious losses caused by the disaster, occurred in the Nenjiang River valley and the Songhua River valley. Greater flood peak occurred three times in the trunk stream of the Nenjiang River due to the floods occurred in its tributaries one after another. At Jiangqiao Hydrometric Station, the water level was 141.90 m and the rate of flow was 12000 m³/s. The flood is ranged to a catastrophic one which occurs once in 50 years. Ranged to a catastrophic flood at Qiqihar Hydrometric Station that occurs once in 400 years, its water level, 0.89 m higher than the former all time high, was 149.30 m and the corresponding rate of flow was 14800 m³/s. The water level that exceeded the all time high lasted for 7 days. At Harbin Hydrometric Station, the water level, 0.59 m higher than the former all time high, was 120.89 m and the corresponding rate of flow was 17400 m³/s. The water level that exceeded the all time high lasted for 9 to 10 days. The flood here is ranged to a catastrophic one which occurs once in 150 years. The flood of the Nenjiang River damaged 456?10⁴ hm² of crops and 1.15 million rooms and the direct loss of economy exceeded 30 billion yuan (RMB). The main reasons of the flood are the great rainfall, the long flood season, the unreasonable land use, the regional ecological environment degradation and the lack of water control projects. It is obvious that the following measures are greatly needed: the comprehensive management of the river valleys; making development planning of the river valleys; strengthening water conservancy projects, especially the water control projects; the development of agriculture based on ecological security.