Abstract: The weld pool oscillation can be excited by fluctuating welding current in thin plate TIG welding.When the fluctuating current frequency is identical with the natural frequency of the weld pool,the resonance of the weld pool is produced so that the arc length and the arc voltage will be changed at same time.The variable parts of the arc voltage are filtered,amplified and monitored by a computer.It is found that during the weld pool resonance,the amplitude of variable parts of the arc voltage becomes maximum then it is decreased obviously.Using the weld pool resonance as singnals to control the weld full penetration is possible.In this paper,the measuring system consisting of a fluctuating current power source and a computer monitor unit is introduced.The regularity of the weld pool resonance and its influence factors are studied.There is a good corresponding relationship between the weld pool size and the weld pool resonance frequency in stationary arc and butt welding.Using weld arc asa sensor through the weld pool resonance signals to control the weld full penetration,there are good repeatability and high accuracy.The method developed in this paper offers a promising prospect for solving the weld full penetrationadaptive control.