Abstract: On the basis of microcomputer controlled IGBT inverter power source,the fuzzy and PID control system of droplet transfer frequency was developed in CO₂ welding process.The system structure,control principle and control algorithm were introduced in this paper.By adjusting the wave parameters and thereby changing arc time,droplet transfer frequency was controlled.The fuzzy control method was adopted for bigger error of droplet transfer frequency,while the PI control method was adopted for smaller error.The control parameter self-study function was realized in fuzzy control process.Good dynamic responsibility and static precision were ensured when welding condition and welding parameters were changed.Welding experiments showed that this system ensured the stability of droplet transfer frequency by automatically adjusting wave parameters when the wire extension varied in welding process,and the control effect didn't depend on the preset value of the control parameters.