Taking the microgrid in a certain area in Southwest China as the research object, an optimal allocation method considering unit cost and self-powered reliability capacity is proposed. First, a microgrid energy management strategy for photovoltaic-storage-diesel generators is designed. Then, with the capacity of each device as the optimization variable, the unit cost is set as the target, and the system self-power supply reliability is the constraint. The optimization model is modeled using a Particle Swarm Optimization (PSO) algorithm. Taking a certain area in Southwest China as a practical example, the minimum unit cost of the microgrid system and the optimal target configuration scheme are obtained, and the effectiveness of the proposed method is verified. This work is supported by Science and Technology Project of State Grid Corporation of China (No. SGSW 0000GHJS1900106).