Abstract: Small distributed energy system refers to capacity below 50 kW,providing cold,hot,electric and hot water loads on site. Based on DeST software,this paper analyzes a model in Dalian city,and obtains the cold,heat,electricity and domestic hot water load of the building. By determining the hourly internal and external thermal disturbances of buildings during the working days and weekends,the hourly load of typical summer and winter days are calculated. Taking the annual total cost and annual CO₂ and NO₂ emissions as the objective function,the Pareto optimal solution set of system equipment capacity allocation was obtained based on the balance of energy flow and equipment performance. The optimal capacity allocation is obtained by TOPSIS optimization method. Finally,compared with the traditional subsystem,the optimal solution set has the advantages of energy saving and environmental protection.