Abstract: In this paper,a special welding power source with the characteristic of constantvoltage controlled by microcomputer has been set up and used as an experimental apparatus.A new method of on-line detection of welding penetration through arcvoltage has been approached by adding an instantaneous pulse at regular intervals tothe normal welding current to excite the oscillation of welding pool in stationary TIGarc welding.On the basis of signal analysis,the relationship between the frequency of welding pool oscillation and welding penetration is studied and a mathematical model,the welding pool vibration equation,is correspondingly established.Using the finite difference equation to solve the eigenvalue problem of welding pool vibration system,one can get the numerical solution of the said relationship.Experimental results of 1Cr18Ni9Ti,20#,and 16Mn steel welding coincide well with those by theoretical calculation.