Abstract: Aiming at the interfacial fracture failure of resistance spot welding structure of hot galvanized steel DP600, the tensile failure modes of DP600 spot welding with different thickness(0.8 mm and 1.5 mm)and length of the nugget (5-8 mm)were studied through statistical analysis method.The relationship between the probability of the interfacial fracture mode for DP600 welding spots and the thickness and the length of the nugget was also established. Then the critical specimen thickness was determined and the influence of the length of the nugget on the critical specimen thickness was investigated.The result shows that the thickness and the length of the nugget have great effect on the interfacial fracture mode of DP600 welding spots.And the critical specimen thickness linearly increased with the length of the nugget.The research result provides a theoretical guidance for the optimization of the structure of high strength steel resistance spot welding in automobile body.