[Objective] To analyze the capacity of green roofs with different substrate layer thickness to hold rainwater and reduce the runoff.[Methods] A series of green roof assemblies were constructed, and artificial rainfall experiments were carried out. Under the different condition of substrate layer thickness and simulated rainfall intensities, rainfall-runoff process and rainwater retention and reduction capacity were studied.[Results] There is a relationship between runoff processes of green roofs and different substrate layer thickness; The thickness of substrate layer of green roofs has exponent relation with rainwater retention rate; Rainwater reduction rate has no obvious relation to rainfall intensity, and it mainly depend on the substrate layer thickness.[Conclusion] Green roof can reasonably reduce the peak flow, total volume of runoff and delay the runoff generation, and the capacity increased with the increase of substrate layer thickness.