The combustion system of circulating fluidized bed boiler has complex nonlinear characteristics, so accurate mathematical model is very important for its effective control. In this paper, the factors affecting the dynamic characteristics of bed temperature and steam pressure are analyzed, and a dynamic model between coal feed and primary air volume and bed temperature and steam pressure is established. The parameters of the model are identified by least square method, and the accuracy of the model is verified by simulation. The results show that the model can reflect the dynamic relationship among coal feed, primary air, bed temperature and steam pressure. This model provides a theoretical basis for the optimal control and stable operation of CFB boiler combustion system.