Abstract: Based on the flood data of October 2024 in Hainan Island, taking Wanquan River Basin as the research object, this paper analyzes the extreme rainstorm floods in the basin caused by No. 2420 Typhoon "Trami" remnant vortex from the aspects of atmospheric circulation, rainstorm causes, flood process, flood composition, reservoir regulation, storage and peak shaving, so as to provide reference for local disaster prevention, mitigation and relief, disaster recovery and post disaster reconstruction. The results show that:① The "Trami" remnant vortex moves little or even stagnates in situ, which is jointly affected by the cold air moving southward, and is the direct cause of extremely heavy rainstorm in the upper reaches of Wanquan River and large floods in the whole basin. It has the characteristics of large process rainfall, strong extremes and concentrated falling areas;② This rainstorm flood has the characteristics of large flood magnitude, long over alarm duration, and breaking the historical record;③ The floods are mainly from the Ding'an River and above the Niululing Reservoir;④ The flood control and peak shaving effect of large-scale reservoir projects is significant.