A dynamic two-phase imbalance mechanism model of pressurizer in pressurized water reactor (PWR) nuclear power plant was built based on some reasonable simplifications and basic assumptions. The equations of energy and mass conservation are used in obtaining a mathematical model for pressurizer operation. The pressurizer is divided into two regions, steam region and liquid region but not necessary in equilibrium with each other. Considering the influence of the spray flow, surge flow, safety valve and heater, the model of pressurizer pressure control system is established by MATLAB/Simulink in this paper. For the large overshoot, response delay and poor stability exist in the pressurizer pressure control system, an advanced model based on PID control algorithm is proposed. The simulation results were compared between two different data obtained by foreign PWR nuclear power plant simulator. All the compared data are quite similar in simulation result, hence it proves the simulation result quite accurate and yet the mathematical model obtained has proved its working principles successfully.