Abstract: Vacuum diffusion welding experiments were carried out for low activation martensitic steel at different parameters, the influence of welding temperature and pressure on microstructure evolution, mechanical properties of welding joints were studied by comparing the microstructure, the interfacial binding rate and the tensile strength of welding joints. The results show that martensitic has transformed to austenite in the diffusion welding process, and the phenomenon is more clearly with the increase of welding temperature. Atomic diffusion effect can be enhanced by improving the welding temperature or pressure in a certain range, as well as the mechanical properties of the welding joint. The tensile strength of welding joint reaches 1 013 MPa at 1 000 ℃ while the welding pressure is 20 MPa and welding time is 120 min, and interface condition is well-bonded.