The impact of the "four colliding" typhoons has become the focus of attention in the disaster prevention and reduction system in the Taihu basin. However, the risk impact of wind, storm, tide, and flood brought by it on the safety of flood prevention is still unclear. It is considerable to carry out the analysis of rainstorm, flood and its movement caused by the typical typhoon of "four colliding".It takes the two typical typhoons "Fitow" and "In-Fa" since 1990 as samples, and other typhoons with two or three colliding as references. Based on hydrological and engineering data, the impact of "four colliding" typhoons is compared. The results showed that: (1) The "four colliding" typhoons all caused extreme rainfall, with a large total amount and wide range, resulting in the rainfall that was more than 70% higher than the average of the two and three colliding typhoons. The higher part was basically the same as the rainfall caused by typhoon Morakot in 2009. (2) The water levels of the Taihu Lake has risen by the second and third place respectively since 1949. In particular, "In-Fa" caused numbered floods, and the maximum tilt of the Taihu Lake was more than 1.00 m. The highest water (tide) level of 33 sites in the river network broke the historical records, and the high water levels lasted for a long time, which seriously threatened the safety of regional flood prevention. (3) It was dominated by storage during "Fitow", and then the drainage capacity of backbone projects was significantly enhanced. During "In-Fa", the flood movement pattern changed to both storage and drainage, with the storage and drainage ratio changing from 1:0.6 to 1:1.1, and the proportion of drainage increased. (4) During the rainstorm of the two typhoons, the drainage was obviously restricted by the astronomical spring tide jacking. After the rainfall stopped and the astronomical spring tide passed, the drainage of the backbone projects increased significantly, especially the Yangtze river to the north and the Huangpu river to the east. (5) Although the drainage capacity of the backbone projects has been significantly strengthened, it still needs to be further improved in the face of the "four colliding" typhoon and the transfer of regional flood disaster risks. The research results could provide a reference for the scientific defense of the "four colliding" typhoon in the Taihu basin in the future.