Abstract: A device without power is designed to detect plasma cloud to judge the keyhole information in plasma arc welding.According to theory of plasma sheath,there is a negative electromotive force,called sheath voltage,on the surface of the probe located in plasma cloud.Detecting plasma cloud with a probe,experiments proved that the detecting sig-nal is very closed to sheath voltage,which is related to temperature and plasma component of detecting spot.Therefore,location of tungsten probe,welding velocity and welding current which influence the temperature of plasma cloud are major factors that affect the detection signal.It proved that with different location of probe detection signal is very different.And sometimes no signal can be detected because of bad location of probe.There is different optimal location of probe to different welding parameter.In tile optimal location,the max detection signal can be detected.