Pneumatic amplifier is not only the commonly used independent electrical attachment but also one of key components of the valve positioner. Here, a mass flow mechanism model of the typical pneumatic amplifier is presented, and the method of parameter identification is proposed. Firstly, according to the working principle and structural characteristics of the pneumatic amplifier, the mass flow model of the pneumatic amplifier is established by using the principle of thermodynamics and fluid mechanics. Secondly, a new sub model is established for the important equivalent orifice area in the experiment, and the parameter identification is carried out. Finally, the model is verified by the experimentalplatform of the control valve. Results show that the accuracy of the model is up to 90% compared with the actual typical pneumatic amplifier.