In industrial loop reactors for catalytic propylene polymerization, the reactor pressure can influence the polymerization rate, feed flow rate, and product properties, especially safe operation of the reactor. To predict the pressure change during polymerization and check the performance of pressure-relief devices in response to emergency accidents, a dynamic model was developed based on mass and energy balances as well as thermodynamics and kinetics. In this model, the pressure was related to the temperature by a state equation for high-pressure liquid. The pipeline equations of the reactors described by the conservation of mechanical energy were incorporated into the model to better predict the pressure. In addition to the pressure, the model can also predict other variables, such as temperature, slurry density, and solids holdup in the reactors. Dynamic data from an industrial plant were used for model validation. Furthermore, the application of the model was demonstrated by simulating several typical emergency accidents.