Abstract: On the basis of the characteristics of pulse plasma arc welding, a feedback control system has been developed for controlling tne penetration of welds. A phototube mcunted under the back of the weld received a light from efflux plasma out of the key-hole and transferred it into a photoelectric pulse signal. After the photo-electric pulse signal being amplified, integrated and then compared with the given voltage, a feedback control system would give an electric signal which could automatically adjust the pulse duration of the welding current and so the duration of the key-hole. Therefore a close loop control of weld penetration could be fully realized.