In an independent regional power grid, the optimal coordination of renewable energy resources such as wind and solar becomes the key to making full use of the energy storage capacity of hydropower. In this paper, a medium to long-term optimal operation strategy is proposed for IRPG in the dry season based on the statistical characteristics of wind-solar power and the long-term plan of hydropower. The objective of the work is to minimize the difference between the monthly water consumption of hydropower stations and the predicted, considering the constraints of water flow and daily average battery energy storage fluctuation. Because of the pessimistic attitude of dispatchers to forecasting results, a correction coefficient is used to modify forecasting outputs. Finally, simulation results show the advantages of the proposed optimal strategy over the strategy without optimal operation in ensuring the water level of reservoir, improving the generation efficiency of hydropower, and avoiding the risk of no water to power generation in hydropower stations at the end of dry season.