According to the design requests of certain heavy truck, this paper introduces a design scheme of pneumatic AMT automatic shift system, which focuses on the structure characteristics and working principles of the three-position cylinder controlled by two solenoid valves. And the mathematical model of the three-position cylinder controlled by two solenoid valves is established based on pneumatic principle. The dynamic model of this mechanism was built by AMEsim software. The variety of pressure and stroke of intake/exhaust chamber was simulated under different air pressures. The theoretical basis is provided to design the AMT election-shift actuator.