Aimed at the island microgrid integrated with wind turbine， photovoltaic， diesel generator， energy storage， and desalination plant， a multi-objective optimal design model considering the allocation ratio of net present cost（NPC） in the full life cycle， the utilization of renewable energy and the pollutant emission level is proposed. The optimal number of distributed generators and the optimal capacity of energy storage are determined as optimization variables. Based on the established control strategy， a floating elitist preserved genetic algorithm is used to solve this problem. The island microgrid is designed and the simulation results are compared with the HOMER optimization results to illustrate that the proposed method is more reasonable and feasible than that of HOMER.