Abstract: Hybrid generation of multiple energy sources is an effective way to promote the accommodation of new energies. It is of great significance to enhance the synergy of different energy sources by determining a reasonable size of the renewable power plant within the hybrid system, thereby enhancing the system’s complementary performance. However, the traditional capacity configuration model for the hybrid system has the disadvantage that the complementary operation process is not accurately simulated. This paper proposes an optimal sizing method on the basis of simulating the refined operation process of large-scale hydro-photovoltaic（PV） complementary power plants to determine the PV size. First, a long-term optimal operation model for the hydro-PV system is modeled under each possible PV size, which determines long-term water and energy allocation strategies. Second, a short-term optimal operation model for the hydro-PV system is established to determine a daily power generation plan and the corresponding PV curtailment rate, constraint by the long-term operation strategy. Finally, the optimal PV size is determined based on a cost-benefit analysis model, aiming at maximizing the net present value of the PV plant over its lifetime. Zhongyu hydro-PV complementary project is selected as a case study. Results show that:（1） when the installed capacity of PV is below 500 MW, the curtailment rate of PV remained within 2%, but the curtailment rate of PV rises rapidly with the increase PV size；（2） when the feed-in tariff is 0.1 CNY/kWh, the net present value of photovoltaic power plants is always negative, and the economy performance is poor；（3） when the feed-in tariff is 0.353 CNY/kWh, the optimal PV installed capacity is 1 900 MW integrated with a hydropower plant with an installed capacity of 1 030 MW, and associated net present value of the PV plant over its life-time ranged from 46.2 to 54.6 billion CNY when the long-term variability of the reservoir inflow is considered.