In order to assess the available quantity of roof rainwater in mountainous urban,the paper took an urban residential district in Chongqing as the study area,analyzed the rainfall characteristics in Chongqing and utilized the improved runoff coefficient method to assess the available water of roof rainwater in typical year,and also analyzed the influence of rainfall characteristics on available water;it used the method combining design rainfall and rainstorm intensity to determine the scale of reservoir,and made which satisfy the requirements of runoff control, flood control and waterlogging reduction.The research showed that the annual collectable water of roof rainfall is 267,951.8, 247,421.3 and 198,917.9 m3 when the annual rainfall is 1198.9,1104.4 and 962.7 mm respectively.The utilization potential of rainfall resources is tremendous.The rainfall amount has greater influence on available water,while the effect of rainfall intensity on collectable rainwater is very little; the total scale of reservoir in the study area is 9,312 m3,the control rate of roof surface runoff reached 86.4%.The paper can provide reference of annual roof runoff quantity, it for the design scale of reservoir in the system of rainfall utilization at mountainous urban.