Abstract: Magnetic fluid dynamics theory was introduced to investigate the features of electric field,induced magnetic field,and electromagnetic field with analytical method.Based on magnetic force distribution,fluid flow laws in spot welding nugget were analyzed.Results showed the magnetic force only acts in symmetry plane through axisymmetrical center of electric electrode, and causes liquid metal to flow in the symmetry plane only.Molten metal makes symmetrical rotation flow in four close cells in the symmetry plane.The flow initiates at faying surface,and the maximum velocity also appears at faying surface.All these conclusions will offer important simplifications for further in-depth research of multi-physical field coupled process during resistance spot welding with finite element method.