Abstract: The penetration depth is one of the most important quality parameters in partial penetration laser welding,and therefore real time monitoring for penetration depth becomes extremely significants.A system was built for real time monitoring the penetration depth of laser welding by measuring and analyzing the plasma optical signal induced by laser.The experiments and analyses show that if increasing the laser power or reducing the speed,the penetration depth is increased and the plasma optical intensity is also increased.A characteristic frequency component exists in the signal from the amplitude spectrum analysis for a whole weld seam which is welded under the same welding parameters,what's more,the characteristic frequency falls as the welding heat input increases.When an 8% fluctuation in the penetration depth is caused by random unstable factors in the same weld seam,the optical signal amplitude decreases rapidly,and there is no characteristic spectrum line in the corresponding signal amplitude spectrum.