In order to solve the problem of low photovoltaic absorption rate after water-solar complementation, the water-solar intra-month complementary scheduling rule was proposed for the first time on the basis of the water-solar intra-day complementarity rule. The complementary characteristics of hydropower and photovoltaics were fully considered by the scheduling rule, and the adjustable performance of cascade reservoirs was adapted to the photovoltaic output curve. Based on the water-solar complementary scheduling rules, taking the cascade power station in the Tibet section of the upper reaches of the Lancang River as an example, the calculation of the output process of the cascade power station and the photovoltaic consumption rate were carried out after complementation. The analysis results show that after the water-solar complementation, the total power generation of hydropower and photovoltaics increases from 55.835 billion kW·h to 59.189 billion kW·h, and the power generation gain after complementation is 3.354 billion kW·h, indicating a 20% increase of photovoltaic absorption rate.