Abstract: Using high-speed photograph,the change of plasma spring during the forming process of keyhole and the relationship between plasma spring and dimension of keyhole had been studied.The results show that plasma spring gets stronger and spring angle gets bigger with the forming of keyhole;plasma spring gets weaker and spring angle gets smaller with the increment of keyhole dimension.By analyzing the relationship between the plasma spring and keyhole dimension,a simple but applicable circuit with tow probes is designed to judge the state of keyhole based on the electric character of plasma spring.A new method of controlling the stability of keyhole was proposed.