During the tunnel boring by shield machines, compressed air is used to control the pressure balance at the working face so as to stabilize the working face and to avoid ground subsidence or ground heaving. Taking the shield machines manufactured by Herrenknecht AG as example, the paper presents the equilibrium mode and application scope of the control of the working face by means of compressed air. The paper analyzes on the features, requirements, components and working principle of the compressed air control system, as well as on the setting of the air pressure, the modes and calculation methods of the air pressure controllers. The paper also makes a detailed discussion on the manner of the adjustment of the parameters of the compressed air control system and provides some recommendations.