A passive electrical probe detection device is employed to detect the electrical signal of plasma in the process of YAG laser welding for A304 stainless steels, and a high-speed CCD camera device is synchronously triggered to record the plasma plume shape. The high-speed camera images of plasma and the electrical signal in time domain are compared and analyzed, and it is found that the fluctuation characteristic of electrical signal is basically consistent with that of the plasma plume shape. The electrical signal with a certain time length is analyzed via its power spectral density (PSD), and it is found that the frequency of the characteristic peak in the PSD plot has a good agreement with the fluctuation frequency of plasma plume shape. The results indicate that there exist strong correlations among the plasma plume shape fluctuation, the fluctuation characteristic of electrical signal and the behavior of keyhole. This kind of fluctuation characteristic is affected by welding parameters, and the fluctuation frequency decreases with the increase of laser heat input.