Abstract: Through the ultra-fining pretreatment on the surface of 40Cr and 0A steels, the dynamic diffusion concentration distribution curves, dynamic diffusion coefficient and diffusion activation energy of substitutional atom Cr in the isothermal superplastic welding process of both steels were measured and calculated by means of the electron probe of modern physical and chemical detection method and the least square method of mathematical analysis. The experimental results indicate that the dynamic diffusion activation energy of atom Cr in the isothermal superplastic welding process of 40Cr/0A steels is closer to the activation energy of matrix phase grain boundary self-diffusion, and the atoms at two sides of interface mostly diffuse through the grain boundary, in which the dynamic diffusion coefficient of atom Cr has increased by two orders of magnitude comparison with conventional diffusion.