Joint operation for multi-reservoir systems leads to synergistic gains. This study aims to explore and attribute the driven mechanism of synergism from joint operation of a multi-hydropower system. It quantified synergistic gains in spatial, temporal, and interannual scales by establishing and solving an individual and a joint reservoir operation model. It then proposed an attribution method for identifying the contribution of water released and water head to synergistic gains using total differential equation. Results of the case study of the downstream Jinsha River cascade and Three Gorges cascade reservoirs during the drawdown season show that: (1) synergistic gains generally occur in May and are mostly generated in Xiangjiaba and Gezhouba; (2) joint reservoir operation is driven by the rapid drawdown policy of Xiluodu and the Three Gorges in early May, which lowers down their water head and gains for most cases; (3) the main contribution factor to synergistic gains of Xiangjiaba and Gezhouba are water released and water head, respectively; and (4) the influence mechanisms of synergistic gains of Xiangjiaba and Gezhouba are jointly determined by their storage and power release capacity conditions. The study provides new insights for analyzing synergism of joint hydropower operations.