Abstract: The weldability of M390 high carbon martensitic stainless steel and 304 austenitic stainless steel has been investigated by flash butt welding method. Firstly, the mechanical properties of the welded joint utilize the room temperature tensile experiment, the microhardness test is characterized. At the same time, scanning electron microscope(SEM), Energy Dispersive Spectroscopy(EDS) and X-ray diffraction(XRD) were used to analyze the microstructure morphology, element diffusion and phase composition of each region of the welded joint. The experimental results show that by appropriate welding parameters, M390 and 304 can form a macroscopically beautiful weld, the maximum tensile strength is 480.62 MPa, and the elongation is 6.2%. The weld metal is composed of martensite, austenite, M₂₃C₆ and M₇C₃. The type of carbide on M390 side and in weld metal is (Cr, V) carbide. Different positions of M390 heat-affected zone are affected by different heat input, resulting in different aggregation and growth of carbides in different positions of M390 heat-affected zone. The carbides in the coarse-grained heat-affected zone of M390 aggregate and grow, and new carbides are continuously precipitated, the carbides exist in two types of morphologies: elongated and granular. The carbides in the fine-grained region are small in size and granular in morphology. The welded joint is brittle fracture, and the fracture position is in the weld metal, which is related to the weld metal structure and the diffusion of elements in the welded joint.