Global river models are crucial for simulating and forecasting floods, especially in regions lacking dedicated flood forecasting systems. Reservoir operations significantly affect flow simulations, impacting forecast accuracy. In this study, we improved a storage-outflow-type global reservoir operation scheme of the global river hydrodynamics model CaMa-Flood, which calculates reservoir outflow based on reservoir storage. First, a non-linear storage-outflow function using convex and concave curves was implemented to enhance flood control and reduce the number of required parameters. Second, a gradual transition in the storage-outflow function was proposed to avoid simulated outflow instability. Third, the outflow during the water-use phase was recalibrated to reflect water availability in dry periods, rather than using annual mean discharge. A case study involving 85 dams in Mainland Southeast Asia was conducted to evaluate the efficacy of the three key modifications by comparing the proposed reservoir operation scheme against three degraded schemes. Using a linear storage-outflow function caused 43% of the 61 reservoirs with a flood control capacity greater than 100 million cubic meters (MCM) to overutilize their flood control capacity during the non-flood water-excess phase. The proposed non-linear function reduced the number of such reservoirs to 27%. In addition, the gradual transition of the storage-outflow function eliminated unrealistic outflow oscillation. Furthermore, parameter estimation for water-use outflow based on reservoir water-use capacity and dry period river discharge enabled better utilization of reservoir capacity during dry seasons. Overall, the proposed reservoir operation scheme holds promise for enhancing the accuracy of global flood simulation within global river models.