Abstract: Keyhole stability during plasma arc welding welding process is critical to the process stability and weld quality.A non power supply probe was developed to detect the plasma cloud voltage,which consists of plasma arc voltage gradient and plasma sheath voltage.Based on plasma sheath theory,the keyhole state information,keyhole establishment and collapse could be predicted by the plasma cloud voltage which changes according to the length and angle of plasma cloud.The signal characteristic and the influences of welding materials,probe location,welding current and plasma gas flow on the signal are discussed.Experiments verified the effectiveness of the method for predicting the keyhole information and joint penetration state.