A air supply system which could supply compressed air with constant pressure and humidity was developed. The system was mainly composed of clean air units, pressure subsystem and humidity subsystem. The proportional flow valve and throttle valve of the system were selected. The humidity subsystem including dry air branch and wet air branch was designed. The humidity of compressed gas was controlled by changing the flow ratio of dry air branch and wet air branch while maintaining constant pressure through PID closed loop. The experiment shows the system can keep constant pressure and humidity in the range of 0~0.2 MPa and atmospheric pressure dew point -15~20 ℃. The maximum flow rate of air supply can reach 79.6 L/min (ANR). This system provides reference for the development of constant pressure and humidity gas supply system.