Reasonable capacity planning of wind farms, photovoltaic power stations and energy storage is a prerequisite to ensure the economy and reliability of a wind-photovoltaic-storage hybrid power system. In this paper, a form of gravity energy storage relying on mountains is proposed. Combined with a wind farm and photovoltaic power station, an optimal capacity planning model of a grid-connected system is established with the minimum costs of the system as the optimization goal. Then, evaluation indices such as the complementary characteristics of wind and solar, the loss rate of power supply and the contribution rate of the system are put forward to evaluate the planning results. In the example, based on the time-of-use electricity price, the capacity planning models considering different energy storage forms, such as gravity energy, battery energy and compressed air energy storage, are simulated and compared. The rank-sum ratio evaluation method with an entropy weight method is used to rank and evaluate the results, and the validity and scientific nature of the proposed model is verified. This work is supported by the National Natural Science Foundation of China (No. 52177110), the Hunan Science and Technology Innovation Platform and Talents Program (No. 2019TP1053), and the Science and Technology Project of State Grid Hunan Electric Power Company (No. 5216A220000F).