Under the conditions of no evapotranspiration and soil water hysteresis, a simplified slope model was established for single-layer homogeneous and isotropic soil profiles on a trapezoid slope with infinite depth for different initial water contents. The Hydrus software, which was used to numerically solve Richards’ equation, was used to investigate the effects of slope gradients on the Horton runoff generation process during temporally and spatially uniform rainfall events. The results show that the runoff on the slope decreases with increasing slope gradients; increasing initial water contents may reduce the influence of slope gradients on runoff; and the relationship between runoff and slope gradients can be well approximated using power functions.