A radon concentration control system for a radon chamber (2.84m3) was developed. The radon concentration in the chamber can be controlled by adjusting air exhaust flow rate through the method of Pulse Width Modulation(PWM). A controlling cycle is 10 minutes

with a device to quickly supply and discharge radon. A solid radon source was used in the radon chamber. The system was controlled by computer. The equilibrium time of radon concentration was less than 4 hours. The stability of long time control was better than 2%.