Abstract: Friction stir keyholeless spot welding (FSKSW) is an efficient manufacturing method for joining dissimilar alloys, which can dramatically offer high mechanical joint strength. Lap joints between AZ31B Mg and galvanized steel DP600 were made by FSKSW, in which the orthogonal tests were used to determine main influencing factors and the optimal scheme. Under the optimal process, the tensile load of the joint reaches 10.1 kN. Based on the analysis of the macro-micro morphology and the distribution of the interface elements, the effects of different process parameters on the diffusion degree of the elements were discussed. The results show that:(1) there is mechanical interlocking structure of hook shape in the stirring area, forming solid mechanical connection; (2) in the diffusion region, there is element diffusion occurring in the interface of Mg-steel with the diffusion layer thickness being 8 μm; (3) in a certain range, increasing the rotation speed of the tool, the vertical descent of the shoulder and the extended length of the pin can promote the elements diffusion in the interface and improve the connection performance.