The impact of “four colliding” typhoons involving a combination of heavy wind, strong storm, high tide and serious flood colliding has become the focus of attention in the disaster prevention and reduction system in the Taihu Basin. However, the impact of the wind, storm, tide and flood on the safety of flood prevention is still unclear. It is important to analyze the rainstorms, floods and their movement caused by the typical “four colliding” typhoons. This study uses the typhoons “Fitow” and “In-Fa” since 1990 as examples and other typhoons with two or three colliding events as references. Based on hydrological and engineering data, the impact of “four colliding” typhoons is compared. The results showed that: (1) All of the “four colliding” typhoons caused extreme rainfall, with a large total amount and wide range. This resulted in rainfall that was more than 70% higher than that caused by two or three colliding typhoons. This was essentially the same as the rainfall caused by typhoon “Morakot” in 2009. (2) The water levels in Lake Taihu have risen to 2nd and 3rd place respectively since 1949. In particular, typhoon “In-Fa” caused numerous floods, with the maximum water level in Lake Taihu rising by more than 1.00 m. The highest water levels recorded at 33 sites in the river network broke historical records, and the high water levels persisted for an extended period, posing a significant threat to regional flood prevention efforts. (3) Storage dominated during “Fitow”, after which the drainage capacity of the backbone projects was significantly enhanced. During “In-Fa”, the flood movement pattern changed to include both storage and drainage. The storage-to-drainage ratio changed from 1∶0.6 to 1∶1.1 and the proportion of drainage increased. (4) During the rainstorms caused by the two typhoons, drainage was obviously restricted by the astronomical spring tide. Once the rainfall had stopped and the spring tide had passed, the drainage capacity of the backbone projects increased significantly, particularly in 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, further improvement is needed in the event of a “four colliding” typhoon or the transfer of regional flood disaster risks. These research results could inform the scientific defence against the ‘four colliding’ typhoons in the Taihu Basin in future.