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Based on the principles of mass, energy conservation and state equation for ideal gas, operation performance of two-stage thermal energy storage system of the first 1MW solar tower power plant in China was calculated and analyzed using numerical simulation method. The results indicated that the pressure of nitrogen in oil tanks slightly changes as a function of temperature and volume variation, thus it is not necessary to control pressure by nitrogen recharge and freeing in normal operation. The steam mass flow rate of discharging process is dependent on pressure variation rate of steam accumulator. Steam mass flow rate increases rapidly to the controlled maximum while pressure varies with time as exponential function. For the case of pressure decreases with time as linear function, the steam mass flow rate increases with time. The steam accumulator needs periodic water supplement because a small quantity of water was lost after every cycle of charging and discharging process.