In order to evaluate service benefit of torrential rain forecasting quantitatively, a mathematical model was build with forecasting accuracy (referred as V), percentage of coverage (referred as P), time validity (referred as N) and damage preventability (referred as Z). Service samples of torrential rain forecasting proved the practicability and maneuverability of the model. Torrential rain forecasting coverage, validity of service and accuracy are in direct proportion to service benefit which is affected by the above three factors, respectively. The stronger capability of preventing weather disaster is, the more benefit is gained from forecasting service. Therefore, torrential rain forecasting service is a kind of information whose value lies on the decision-making and defence capability of the applying information colony.