Under the dual pressures of global climate change and rapid urbanization, compound flood disasters resulting from the concurrent occurrence of heavy rainfall and high tide levels have become a significant risk for coastal cities. To investigate the precipitation characteristics in coastal areas and quantitatively evaluate the impact of joint rainfall-tide encounter risks on flood prevention and drainage systems in coastal cities, this study analyzed annual maximum 1-day rainfall and corresponding tide level data from 1960 to 2018 in the Qianshan River basin. The Mann-Kendall test and the moving T-test were applied to identify abrupt change points in rainfall and tide level series. Based on the optimal marginal distributions and Copula functions, joint distribution models of rainfall and tide levels were constructed for the periods before and after the identified change points. The co-occurrence risk probabilities, flood prevention risk probabilities, and their return periods were analyzed for different time periods.The results indicate that: (1) The year 2000 marks an abrupt change point in the tide level series of the Qianshan River basin; (2) Compared to the period from 1960–2000, the co-occurrence risk probabilities during 2001–2018 decreased, while the flood prevention risk probabilities increased. This may be attributed to the rise in tide levels caused by sea level rise and human activities. (3) The return periods of rainfall and high tide level encounters shortened during the second period, indicating an increased frequency of extreme rainfall-tide events and heightened risks of extreme events. Therefore, raising the design standards for tide levels can effectively mitigate flood prevention risks.