The rainfall runoff reduction effect on green roofs was analyzed and tested by comparative rainfall runoff monitoring on the impermeable roof (slope, plane). The evaluation index of the rainfall runoff interception benefit (relative runoff reduction rate, rainfall control rate) on green roofs was studied. The results show that compared with slope and level roofs, the change range of green roof runoff reduction rate relative to level and slop roofs is 20.0%–98.3%, 3.8%–92.3%, the mean value is 48.4% and 34.3% respectively. It is obvious that green roof has better rainfall runoff reduction effect. It can be seen from the single rainfall control effect that, the variation range of green roof rainfall runoff control rate is 36.0% to 99.0%, the total rainfall control rate is 57.6%, which reflects that the green roof has better rainfall control effect. Through comparative study, it can be concluded that the rainfall runoff control rate is more suitable for the design index of green roofs.