Abstract: Numerical simulation of resistance spot welding of high strength DP590 steel and 6061 aluminum alloy was carried out by finite element method (FEM) software SORPAS.Three stages of nugget formation process of steel-aluminum RSW had been analyzed.Meanwhile,the nugget offset phenomenon and dual nugget characteristics caused by asymmetric temperature distribution field feature of steel-aluminum RSW had been elaborated.The macro and micro structure of the weld was examined with EDS,and the dynamic resistance of the welded zone was measured.The results revealed that the melted aluminum alloy spread and diffused on the interface between the steel and aluminum, which led to the formation of fusion-brazed joint.The thickness of intermetallic compound (IMC) is less than 2 μm while the atomic percentage of aluminum diffused in steel are about 40%.