In this study, the relation between keyhole oscillation and depth is analyzed based on the internal pressure balance conditions of the keyhole to allow real-time monitoring of the laser penetration welding process. Then, based on the coupling of keyhole behavior with plasma behavior and consistency of plasma oscillation characteristics with plasma electrical signal fluctuation characteristics, we use a short-time autocorrelation analysis method to analyze the relation between the oscillation period of a plasma electrical signal and weld depth during laser penetration welding of A304 stainless steel and Q235 carbon steel. Results show that the plasma electrical signal's oscillation period increases with an increase in the weld depth, and the relations between the plasma electrical signal's oscillation period and weld depth differ when the welding materials are different. Finally, in a variable heat input continuous welding verification test, we obtain a good correspondence between the short-time autocorrelation analysis results of plasma electrical signals and weld penetration when the welding process is stable, which is consistent with the keyhole oscillation characteristic equation we analyzed.