Abstract: Expulsion is highly undesirable during the resistance spot welding for it decreases the weld quality. An axisymmetric finite element model was creates to study the distribution of temperature and the plastic deformation to investigates interior expulsion happening during the resistance spot welding of AZ31 magnesium alloy, employing a contact resistance model based on the micro-contact theory. Because of high thermal conductivity, low melting point,small specific heat and large linear thermal expansion coefficient of AZ31 magnesium alloy, expulsion is most encountered problem due to the application of high current in short welding time, compared with Al alloy and mild steel. And it is connected with the increased pressure in the nugget due to melting so that the surrounding solid material cannot hold the liquid phase and the molten metal break through the constriction of peripheral boundary, which leads to expulsion.