Wind, photovoltaic, and thermal power generation can form a coupled system through the same grid-connected point, which is a high coordination and low-carbon approach of renewable energy and flexible regulating power source at generation side in northern China. By considering renewable energy ramps and source-load matching analysis, this paper studies the optimal capacity planning of a wind-solar-thermal coupled system to provide reference for coupled system planning. First, the operation model and the uncertainty method of coupled system are briefly described. Then, considering the wind-solar complementary, ramp events, and critical load characteristics, relevant indices are selected and proposed for source-load matching evaluation. After that, considering the constraints of source-load characteristics, matching, and cost, an optimal capacity planning model of wind-solar-thermal coupled system is established. Finally, based on the actual data in Liaoning Province, a case study is conducted to acquire the optimal capacity of the wind and solar generation in the area, and the interaction between the source-load relevant indices and the planning results is analyzed, which provides reference and suggestion for the optimal capacity planning of renewable energy generation.