The output of distributed photovoltaic has great uncertainty. In order to analyze its influence on electric-thermal combined scheduling based on phase change energy storage, traditional evaluating indicator of interval prediction is improved, and a method for evaluating the fluctuation range of distributed photovoltaic output is proposed utilizing improved interval prediction model. Based on electric-thermal combined dispatching model, considering the volatility of photovoltaic output reasonably, a strategy to minimize the total cost of purchasing electricity is formulated. Also, the economy and risk of this strategy under different rated confidence levels are analyzed. Finally, in the simulation, the relationship between photovoltaic output fluctuation and the system economy and risk is analyzed, which provides a reference for the practical application of phase change energy storage system. This work is supported by National Natural Science Foundation of China (No. 51537003) and Science and Technology Project of China Southern Power Grid (No. 090000KK52180027).