One of the main shortages of thermodynamic system analysis in coal fired power plants is the lack of optimal value of initial parameters under present operating condition. In this paper, a mathematical model is given to solve the problem with the systemic analysis method for the first time. Through quantitative calculations of N300-16.7/537/537-3 unit of a certain thermal power plant, the optimal initial pressure is obtained by method of exhaustion. It is the foundation of the optimal control in the whole power plant. The limitation of optimal operation curve provided by manufactory is also analyzed in this paper.