A novel passively electrical device is proposed to detect the electrical signals of plasma in YAG laser welding of A304 stainless steels and a high-speed camera is used to observe the plasma shapes. The results indicate that the time-domain characteristics of signals are different in different welding modes. The theoretical and experimental analyses of the signal characteristics under different welding modes indicate that the plasma electrical signal is influenced by the combination of the plasma effect and sheath effect; the determinant factor resulting in different electrical signal characteristics under different welding modes is whether the keyholes are formed or not.