By using a limited area operational forecasting model and choosing a precipitation weather process caused by a tropical storm in August 1998,comparative forecasting experiments of an improved mass-flux cumulus parameterization scheme taking into account the effects of both updrafts and downdrafts and the original one only includes updrafts used in the model were performanced.The results show that downdrafts contribute significantly to the improvement of the model precipitation forecasts for over 25.0mm.