Season-of-use（SOU） price is an important measure of demand side response（DSR）， and a rational level of SOU price is crucial to its successful implementation. Based on an analysis of the historical data and electricity consumption behaviors， customers' response to SOU price is first described by a price elasticity matrix of demand. The concepts of differential rate of season demand， hydro-thermal electricity exchange index and risk of implementing SOU price are then presented. From the viewpoint of electricity market operation risk， the market benefit of SOU price is evaluated with value-at-risk（VaR） method. Furthermore， a decision model of SOU price is introduced and then solved by genetic algorithm（GA）. Numerical results are finally used to prove the effectiveness of the proposed model.