Abstract: To address the limitation of existing hydraulic methods in calculating ecological flow in rivers, which fail to adequately account for intra-annual variations in water inflow in mountainous rivers, this study takes three cascade hydropower stations on the Liuxi River in Guangdong Province as an example.An improved hydraulic method for calculating ecological flow was proposed, incorporating intra-annual rainfall variations.Based on the appropriate water requirements of aquatic organisms in the river and the monthly distribution of rainfall in the basin, the water depth, flow velocity, and monthly wetted perimeter ratio standards for river cross-sections were determined, thereby the monthly ecological flow for each section was calculated.The calculated monthly ecological flows were compared with the evaluation criteria of Tennant method and the results of wetted perimeter method.The results showed that the monthly ecological flows of the three cascade hydropower stations all exceed the values corresponding to the 10% (fair condition) evaluation standard of the Tennant method, reaching the 40% (good condition) standard during high-flow periods, which helped maintain the stability of the aquatic ecosystem in the river during low-flow periods.Compared to the wetted perimeter method, the monthly ecological flow calculations for the three cascade hydropower stations better aligned with the ecological needs of the river under different inflow conditions.Considering the impact of intra-annual rainfall variations, determining monthly ecological flow values through period-based can achieve refined ecological flow management in river channels.