The electrical signals of laser induced plasma from NdYAG laser welding of A304 stainless steels are detected by a passive electrical probe. The relationship between different laser welding modes and the spectral analysis results of plasma electrical signals are investigated. The waveforms and spectra of plasma electrical signals are analyzed. The relationship between the formation characteristic of laser welds under different laser welding modes and the waveforms and spectra of the plasma electrical signals is illustrated. The results indicate that, under the specified conditions in the experiments, different laser welding modes correspond to different spectral characteristics of plasma electrical signals. Moreover, whether the welding mode is a deep penetration one or not can be identified by the spectral intensity E of plasma electrical signals within the scope of 500～1000 Hz. When E is large enough or exceeds a certain value, the laser welding mode is a deep penetration welding.