To solve the problem of island power supply，an optimal capacity configuration method for dual energy storagesystem with pumped storage and battery storage is proposed． The optimization goals of the system are the lowest economiccost and the lowest carbon emission． Based on the data of an office building on an island in the East China Sea，thesystem model is solved by PSO algorithm based on constraint，and the optimal capacity allocation scheme is obtained byfuzzy decision method． The results show that the economic cost and carbon emission of the dual energy storage system arethe lowest，and the energy efficiency of pumped storage is 9. 7% higher than the pumped storage system; Based on theε constrained PSO algorithm，it is feasible to solve the capacity configuration problem; Increasing the power purchasingrate properly will increase the carbon emission of the system while can effectively decrease the economic cost．