In this study, we took the Tunxi, Dongwan, and Wangkuai watersheds as typical watersheds and studied the effect of watershed subdivision on the Xin’anjiang model’s parameters, including S_M, C_S, C_I, and C_G. We used the simplex algorithm with suitable objective functions to calibrate the parameters of the model. The results of the study show that, with the increase of the number of watershed subdivisions, the homogenization degree of rainfall data decreases, S_M has an increasing trend, C_S has a decreasing trend, and C_I and C_G remain invariant. The number of qualified flood volume and flood peaks and the deterministic coefficient show an increasing trend with the increase of the number of watershed subdivisions. When the area of the sub-watershed is less than 1 000 km², the simulation accuracy improves significantly.