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A simple cost-loss ratio decision model is used to illustrate the relationship between the skill of a weather forecasting system and its economic value. Using the ECMWF ensemble forecasting system as an example, a deterministic forecast case and a probability forecast case are discussed, respectively. The comparative analysis of the relative economic value curve of EPS probability and deterministic forecasts of the ensemble forecasting system shows that the probability forecasting system is obviously superior to a deterministic forecast system with the same quality in value. The results indicate that for a forecacting syctem,it is useful for users only in the case of the cost-loss ratio C/L being in a certain range. It is very important for realizing the maximum forecast value to choose the optimal probability threshold value based on C/L.