Abstract: During Typhoon " Co-may ", the water level of Suzhou River exceeded its historical maximum, posing severe challenges to flood control safety in Shanghai. We analyze the overview of Typhoon " Co-may, " the record-high water level in Suzhou River during the typhoon, and the precipitation characteristics in the areas along both sides of Suzhou River, revealing the causes of this extreme water event. The analysis indicates that the primary reason for the record-high water level in Suzhou River was the rapid increase in channel storage within a short period. The underlying mechanism lies in the extreme heavy rainfall brought by the typhoon, which caused floodwater from both sides of the river to rapidly converge into Suzhou River. Simultaneously, due to the backing-up effect of the upstream base flow and the high tidal level of the outer river, the drainage capacity of Suzhou Creek was severely limited. The combination of these two adverse factors—"rapid inflow and slow outflow"—led to a sharp rise in the water level. By comparing Typhoon " Co-may " with last year’s Typhoon " Kong-rey ", the extremity of the disaster-inducing factors associated with the typhoon’s rainstorm combination is further highlighted. Based on this analysis, we propose measures and recommendations, such as optimizing scheduling plans and engineering measures, to address similar extreme events in the future.