In view of the urgent needs of urban flood control risk management, indicators from aspects of hazard-affected bodies, hazard-forming environments, and hazard-inducing factors are selected to construct an integrated flood risk assessment system based on disaster theory. Then, the AHP-CRITIC method is coupled to calculate the comprehensive weight of each indicator, the grey relational analysis method is used to calculate the flood risk of each unit, and the natural breakpoint method is adopted to classify the risk levels. Taking the main urban region of Sanya City as the research object, a flood risk assessment model is established to carry out the flood risk level zoning. The results show that the proportion of areas with four levels of flood risk from extremely high to low in the main urban region of Sanya City is 5.60%, 13.84%, 33.04% and 47.52%, respectively. The extremely high-risk and high-risk areas are mainly distributed in the lower reaches of the Sanya River and the East River of Sanya, as well as the low-lying contiguous cultivated land on both sides of the lower reaches of the Tangta River. The comparison analysis proves that the flood risk assessment model based on AHP-CRITIC-grey relational analysis is superior to any individual model, which can provide a more reasonable distribution of flood risks. The research results and flood risk assessment model can provide a reference for flood control and disaster reduction planning of Sanya City and other cities facing similar problems.