The total profit of a thermal power plant mainly include electricity and reserve profit. The allocation of unit capacity is one of the important factors affecting the total profit. Thus a day-ahead and intraday two-stage optimal dispatch model to optimize the capacity is proposed. The participation mechanism of Energy Storage (ES) in power dispatching is analyzed and the available reserve capacity of ES is quantified. Then a day-ahead and intraday two-stage optimal dispatch model of a thermal power plant with ES is established to reduce the climbing frequency of units and improve the utilization rate of its generation. The model is processed by Lagrange relaxation, and the capacity and electric energy price and the total profit of power plants are calculated based on the marginal price theory. The model is tested in two scenarios: where ES is installed or not. The results show that the installation of ES at a power plant can improve the utilization rate and the profit of generating units. This work is supported by the National Natural Science Foundation of China (No. 51877154).