Abstract: Penetration information of keyhole pulse plasma arc welding has been correctly and conveniently obtained in real-time by detecting the reflecting plasma-flame voltage with non-power supply probe system.A fuzzy controller,which is realized by Look-up Table method,was developed and simulated on MATLAB fuzzy logic toolbox to control the penetration.In this controller,the peak welding current is controlled by the keyhole existing-time in a period,by means of the feedback of reflecting plasma-flame voltage.The results of experiments demonstrate that the "one-pulse-one-keyhole" strategy could be achieved;the continuous,stable and reliable welding process could be fulfilled;the nice weld beam appearance could be gained by using the control system.