A demand response uncertainty model based on the user demand curve is proposed, which reflects the uncertainty of user response behavior by the interval elastic coefficient, and the uncertain demand curve is obtained. In order to improve the reliability of demand response, an optimal allocation model of energy storage of load aggregator is constructed based on the interval number of user response deviation, and a Monte Carlo simulation is used to solve the model to obtain the energy storage configuration plan. An improvement in the energy storage on demand response reliability is evaluated. PJM historical data simulation shows that the proposed model and algorithm can effectively improve the user's demand response deviation, optimize the load curve, and enhance the reliability of demand response. This work is supported by the National Key Research and Development of China (No. 2016YFB0901100).