In SWAT model, the built in reservoir model was modified based on the characteristics of check dams to set up the check dam module, with which the impacts of check dams on runoff and sediment load in a watershed was studied, and then the model was verified in the simulation of Xiliugou Basin which is one of the ten tributaries in Inner Mongolia. Taking 1980-1990 as the calibration period and 2006-2015 as the verification period, the simulated runoff and sediment load of Longtouguai station fit well with the observed value, and the linear fitting coefficient (R2) and the Nash-Sutcliffe efficiency coefficient (ENS) were both greater than 0.6, which indicates that the built in reservoir mode can be used for the simulation of check dams. The simulation results show that when the other parameters remain unchanged, the check dam has a certain impact on the runoff and can intercept part of it, moreover the check dam can greatly reduce the sediment load. Check dams can affect the confluence process of a basin to some extent, making the simulated runoff larger than the observed value in the months after the flood season. Compared with the empirical formula method, SWAT model can improve the accuracy of the simulation on impacts of check dams on runoff and sediment load in a watershed. In order to make better use of SWAT model for these simulations, the check dam module needs to be further improved.