This paper proposes a wind-photovoltaic-thermal energy storage hybrid power system with an electric heater, which adopts the idea of concentrated solar power plant but omits the expensive solar field, and utilizes the reformed power block from conventional small-scale thermal power plant. From the perspective of thermal power plant reconstruction, the hybrid system aims to minimize the levelized cost of energy and maximize the utilization rate of transmission channels. The capacity optimization is solved by multi-objective particle swarm optimization algorithm. Final optimal capacity is obtained by the Pareto front and decision-making method. Moreover, the probability of exceeding is introduced to correct the theoretical power output calculated by mathematical models and improve the practicality of simulation. Finally, by comparing wind-photovoltaic-thermal energy storage system, wind-photovoltaic-battery system and wind-photovoltaic system, it can be concluded that the proposed system effectively increases the utilization rate of transmission channels and presents a better reliability and economy performance.