An artificial rainfall device and a data acquisition system are designed to study the rainfall-runoff processes of an extensive green roof with matrix of 57.0% ceramsite, 41.5% peat, and 1.5% super absorbent polymer. Based on 37 times artificial rainfall tests, the fundamental processes of rainfall infiltration and runoff are illustrated. The effects of rainfall intensity, matrix depth and initially water content on the rainwater retention features are studied. The results show that the effects of matrix depth and initially water content are significant. The rainwater storage in matrix will increase with lower initially water content and higher depth. A multiple linear regression equation is presented for the delayed time of runoff. It shows that the delayed time of runoff is negative related with rainfall intensity.Meanwhile the delayed time of runoff are affected by initial water content and depth. With the increase of wetting and drying times, the matrix depth tends to decrease, the rainwater storage gradually declines, and the delayed runoff time tends to shorten.