Abstract: According to the measure principle of electric eddy sensor,the basic principle of detecting weld pool temperature with the electric eddy sensor is studied.It is found by investigating on the welding that the relationship between the weld pool temperature and output voltage of the sensor is single valued.The electricity parameters and the structure parameters of the sensor are calculated theoretically,and an electric eddy sensor with a cooling system,which can solve the problem caused by the heat radiation,is designed.The sensor can detect true voltage signal about the changing temperature of the back side of weld pool when using the step traveling pulse TIG welding on mild steel.A new studying method for penetration control is proposed in this paper.The advantages of the sensor are that it doesn't adopt contact measure that can disturb the heat radiation and arc,it is strongly anti jamming,its structure is simple,and its cost is low.