In order to study the flood characteristics of small watersheds under extreme rainstorm conditions, a two-dimensional full hydrodynamic numerical model is used to simulate the flood routing process of a small watershed, with which the hydraulic elements of typical sections under different return periods were calculated, and the flood risk analysis of the watershed was conducted. The results show that the peak flow growth rate corresponding to the return period of 500 a is the largest, up to 51.38%, which drops dramatically to 7.61% when the return period is increased to 1 000 a, because the peak flow growth rate increases first and then decreases with the increase of rainfall return period. Moreover, the maximum water depth increment of each section is smaller than its corresponding peak flow increment, and the flood inundation area of the watershed changes little with the increase of the return period. In order to maximize the social and economic benefits, it is recommended that the peak flow under the return period of 500 a should be adopted for the calculation of aperture size of highway and railway bridges and culverts in small watersheds. The research results can provide some necessary reference for the management of extreme rainstorms and evaluation of flood control in small watersheds.