Abstract: Based on Xin'anjiang Model and EFDC Model, the coupled flood model of mountainous towns was constructed to analyze the flood problem of Tangxi Watershed in Yongkang City, including the assessment of river flood capacity, flood risk analysis of villages and rainfall threshold analysis of flooding villages. The results show that the flood capacity of Tangxi main stream is insufficient, some river channels are narrow and have insufficient flow capacity, which is easy to produce river overflow after rainstorm, and overflow flood will inundate houses and farmland near the river channels, resulting in flood disasters. Among them, the flood control capacity of the river reach near Shanglongkou Village is less than once every five years, and the flood control capacity near Shanghuangqiao and Baokeng villages is less than once every 10 years. When the rainfall reaches 60 mm in 3 h, Shangnongkou Village are high-risk villages, and when the rainfall reaches 100 mm in 3 h, Shanghuangqiao Village and Baokeng Village are high-risk villages. The flood standard of the river near these three villages is improved to once every 10 years. It can greatly reduce the flood hazard of the towns in the Tangxi Watershed.