Probability analysis method is put forword to estimate the longterm operation efficiency of urban concavedown greenbelt which stores and infiltrates rainfall runoff.Case study was also carried out for proper design of the concavedown greenbelt based on the 20year statistical data of the hourly rainfall characteristic parameters in Shanghai area.The results show that this method can ensure the safety of the engineering design of urban concavedown greenbelt,and that the optimal practical concave depth was 80~160 mm,under which the storageinfiltration efficiency could reach up to above 90%.