As large reservoir groups are constructed in the upstream of the watershed，the downstream river discharge increases significantly in the dry season due to the mode of storing water in the wet season and supplying water in the dry season，which is of great significance to improve waterway conditions. Based on hydrological data of the downstream control hydrologic stations of Jinsha River over previous years，we analyze river discharge variations in the dry season during different periods of reservoir group construction and put forward a theoretical method for calculating downstream river discharge amplification under different guarantee rates in the dry season based on reservoir capacity adjustment of reservoir groups. Then we predict quantitatively that the river discharge of Xiangjiaba station under a guarantee rate of 95% is 2 172 m3/s after two reservoirs are constructed in Wudongde and Baihetan. The value approximates the actual statistic value of 2 007 m3/s. Therefore，the proposed theoretical method can provide references for estimating downstream river discharge in the dry season after cascade reservoir groups are built in other rivers.