In order to meet the refined management demand of optimal operation of large-scale cascaded reservoirs and solve the problems of long running time and low computational efficiency, a parallel method based on the Fork/Join multi-core parallel framework is proposed for optimal operation of large-scale cascaded reservoirs. A parallel discrete differential dynamic programming(PDDDP)was designed to describe the parallelization scheme for optimal operation of cascaded reservoirs based on the Fork/Join framework. The long-term power generation optimal operation of large-scale cascaded reservoirs on the Hongshui River was used as a case study. The testing results show that the parallel method can make full use of the acceleration performance of the multi-core processor, significantly reducing the computation time, and improving the computational efficiency. Moreover, the choice of the reasonable scale control threshold of the Fork/Join framework is critical to taking full advantage of parallel computation.