Abstract: Based on the theory of sensing the plasma cloud with a metal probe,an applied multi-probe plasma cloud charge sensor was developed,and its reliability and precision were tested and compared with a designed efflux plasma charge sensor.The plasma cloud charge sensor was applied to detect the status of keyhole molten pool during pulsed plasma arc welding,furthermore, the detected signals were studied.As a result,the detected undershoot of plasma cloud signal can be used as the criterion of the keyhole molten pool's status,and the frequency of the undershoot can be used as the criterion to judge whether the pulse welding is one-pulse-one-keyhole or not.It was shown that the detected plasma cloud signal can reflect the keyhole molten pool's status.