Abstract: In order to study the shear properties and microstructure of 18Ni martensitic stainless steel resistance spot-welded joints for aerospace applications. In this paper, the resistance spot-welding experiment and shear experiment of 18Ni martensitic stainless steel were carried out. Besides, the finite element analysis software was used to simulate the stress distribution of spot-welded joint in shearing process. The results show that the maximum shear resistance of 18Ni stainless steel resistance spot-welded joint is 45.01 kN, and the minimum shear resistance is 36.87 kN. The simulation results show that the stress is mainly concentrated on the spot-welding seam. Especially, the spot-welding seam between the two steel sheets has the maximum stress. After metallographic structure observation, it is found that the micro-structure of spot-welded joint is similar to that of casting. The coarse columnar crystals grow from the edge of the weld seam to the center of the weld seam, and a clear grain boundary line is formed in the center of the weld seam finally. It is also found that the internal structure of the grain is acicular martensite and residual austenite. The grains in the heat affected zone are isometric crystals and the size of isometric crystal is smaller than that of weld columnar crystal. It is also found that there is micro-crack in the weld seam.