A method with forecast service experiences and mathematical-statlstic computation combined is introduced, which can be used to calculate objectively forecast service economic efficiency. The reduced part of the loss after taking countermeasures according to forecast service is considered as the direct economic efficiency of the forecast service. The indexes of various weather risks are determined through user investigation and analytical experiments, and the corresponding computation models are established by means of the statistical method. The method has advantages of being simple, less errors, and easy to use. The practical application of nearly three years proves that it is applicable.