A hollow cylindrical electrode was employed to detect the electrical signal of laser-induced plasma during YAG-pulse laser welding of Al and 45# steel. The theoeritical model has been established for analysis of the plasma region and compared with the experiments.The research results show that the current exhibit a saturation effect in the I-V curves of plasma signal with increasing voltage in the negative voltage region, and the plasma temperature is higher than the vaporzation temperature of welded materials.