Abstract: Based on the strength test, SEM analysis of cross-tensile fracture and TEM analysis of nugget of spot welding joint (RSW) from two kinds of D90 dual-phase high strength steel with approximate strength level and different chemical composition, the mechanism and effect of chemical component of base metal on the strength and toughness of RSW joint of the D90 steel are studied. The results show the more carbon content in base metal is, the less cross-tensile strength and toughness of spot welding joint are. The main mechanism is that the twin martensite forms in the nugget and fusion zone as the increase of carbon content under imbalanced express cooling of RSW, which will lead toughness of spot welding joint to decrease.