To solve the problem of output power fluctuation in gravity energy storage systems due to discrete mass block switching, a power smoothing control strategy based on instantaneous power mirror compensation is proposed. Firstly, the working principle of the slope gravity energy storage system is introduced to analyze the causes of power fluctuation of the system. Secondly, the system structure for suppressing power fluctuation as well as the power smoothing control strategy are designed, and the theoretical calculation of the system structure parameters is carried out by taking the example of a 11 kW slope gravity energy storage system. Finally, a simulation model for slope gravity energy storage system is established, and the transient fluctuation characteristics of the system during the mass block alternation process are analyzed. The results show that by applying the proposed method, the mass block can provide continuous and stable mechanical power during the alternating process, which can better smooth the output power of the gravity energy storage system, verifies the effectiveness of the method.