Hydropower project construction operation will have a negative impact on the surrounding ecological environment.Taking the biggest rivers in Jiangxi province，the Ganjiang river basin as an example，the Indicators of Hydrologic Alteration（IHA） at different stages are calculated according to the measured runoff data of Dongbei hydrological station， and the Range of Variability Approach（RVA） is used to analyze the influence of the construction and operation of Wanan reservoir on the hydrological situation of the river basin.The results show that the comprehensive change degree of Wanan reservoir is 49.24% after operation，which is a moderate change，therefore，the degree of influence on the downstream hydrological situation is moderate.