An advanced plasma detecting system of CO2 laser welding has been developed. The system consists of three sensors, signal processing, A/D data converting and photo-electric coupling units connected with a rapid personal computer. The Photocell Sensor (PS) detects the intensity of the blue light irradiated by the plasma. The Plasma Charge Sensor (PCS) detects the electric density of the plasma plume. The Microphone Sensor detects the sound pressure coming from the rapidly expanding vapor in the keyhole. All of the sensors can exactly distinguish three kinds of welding processes--heat conduction welding, deep penetration welding, unstable mode welding. When the welding parameters are given, the PCS signal depend on the distance between the welding nozzle and the workpiece, the PS signals are correlated closely to the focal point position. Three sensors can be used to control the focal point position (penetration depth) under given laser power and welding speed. In addition, the relation between detecting signals and penetration depth is given. The sensors of the system have features of simple structure, low cost and high sensitivity, which are especially suitable for on-line plasma detecting, quality controlling and off-line plasma analyzing of CO2 laser welding.