The effects of rainfall intensity, rainfall duration, rainfall peak coefficient, and rainfall pattern on the runoff coefficient of semi-permeable roads were systematically investigated via physical model experiments with simulated rainfall events. The results show that runoff coefficient increases with rainfall return period, with the value increasing from 0.093 to 0.377 when the return period is varied from 0.5a to 20a; it also decreases with the increase of rainfall duration and increases with the increase of rainfall peak coefficient; the runoff coefficient under the condition of uniform rainfall pattern is slightly larger than that of Chicago rainfall pattern when rainfall return period and rainfall duration are kept in the same experimental condition. Rainfall return period has the most important influence on the runoff coefficient of semi-permeable roads, followed by rainfall peak coefficient and rainfall pattern.