Abstract: In view of the problems that the proposed large-scale irrigation area project may cause flow reduction and fish ecosystem damage to the downstream river channel of the reservoir after water diversion from the reservoir,The ecological discharge of the 36.9km downstream of Maliba Reservoir in the study reach of Nanwan River was studied by the Q₉₀ method of hydrology and R2-Cross method of hydraulics. Among them, Q₉₀ method calculated the driest monthly flow in a long series of years under 90% assurance rate,R2-Cross method firstly proposed a hydraulic parameter standard suitable for determining ecological flow in this basin according to the characteristics of river width and target fish body length in this basin, which is used to set the protection standard of river fish habitat. These parameters include water surface width, average water depth, wet cycle rate average velocity. According to the calculation results of the two methods, the maximum value was taken as the final ecological flow value, which provided the basis for ecological flow calculation and water resource management after water conservancy project construction.