Abstract: Resistance plug welding was used to weld dissimilar steels. Orthogonal test was used to optimize welding parameters and range method was used to analyze the key factors affecting spatter defects. Then impulse tests were carried out on all resistance plug welded joints. The formation of molten pool, the law of resistance distribution and the electrode were studied. The mechanism of spatter of resistance plug welding was studied in the aspects of pressure and heat change. The results showed that spatter has a great influence on the impact absorbing power of the resistance plug welding joint. The impact absorbing power of the spattered blocking welded joint is greatly reduced. The process of resistance plug welding includes filler heating, filler voltage leveling and nugget. In the three stages of nucleation, the contact resistance increases, decreases and increases again with the welding process and the change of filler shape; at the end of the flattening stage, the instantaneous heat surge is the main reason for the splash on the outer edge of the solder joint; and during the flattening process, the large gap between the two kinds of steel contact surface is the main reason for the splash on the joint surface.