Pneumatic operated spring-diaphragm actuator is the most popular acting device in industrial applications. Precise modelling of pneumatic actuator continues to remain a major challenge because of its nonlinear characteristic. In this work, the characteristics of pneumatic actuator are analysed in details, and then the dynamic model, which considers effects of stem friction and compressibility of air, is developed. Computer simulation and experimental tests have been carried out on the developed model. The results show the model has a better performance.