In a power grid mainly relying on the thermal generator set， the reserve water used to meet the needs of black start has a direct impact on the benefits of peak load shifting. In order to get the largest benefits of the pumped storage power station， the calculating method for black-start benefits and the required water of black-start are proposed. Considering the black-start benefits of the pumped storage power station， static benefits and the constraint of the marginal running position， the calculation model for optimal reserve water is developed. Simulation results of Shandong power grid in China show that the model and method proposed are effective.