The invention discloses a capacity optimization method of a photothermal-photovoltaic-wind power combined power generation system. The combined power generation system adopts a wind power plant and aphotovoltaic power plant for direct power supply, the photothermal power plant is used for peak regulation, frequency modulation and reactive power compensation, so that the installed capacities of the photothermal power station, the photovoltaic power station and the wind power plant are optimized by adopting a particle swarm algorithm under the condition of the lowest construction cost, the minimum annual total power loss cost, the minimum annual total power shortage cost of load with insufficient power generation and the maximum annual total power generation income, the annual power loss rate and the load power shortage rate of a combined power supply are reduced, and the power generation quality and efficiency of renewable energy are improved.