The Three Gorges cascade reservoirs and Qingjiang cascade reservoirs have been completed for cogeneration represented the largest scale cascade hydropower system in the world were formed. The joint operation of these reservoirs are publicly deemed necessary to greatly improve the power output in terms of present reservoir operations. Based on existing reservoir operating rules, two modeling methods of routine and optimal operation were all adopted in this paper to fully access effects of the joint operation. To trace the realistic and potential impacts, the two models were computed using the observed runoff data of the fifty years period from 1951 to 2011, and the potential impacts were identified though the comparative studies between total generation for the two single cascades and cogeneration for the two cascades. Contrast tests showed that there are little room for the improvement of hydropower generation with current reservoir operating rules, although the joint operation can increase hydropower generation levels especially for the refill stage and the release stage of reservoirs.