Abstract: To understand the nature of carbon migration and diffusion in the welding process of 12Cr2Mo1R and S30408 dissimilar metals, the phenomenon of decarburization and carburization layer in the weld of the dissimilar steel after welding is confirmed in this paper through experiments; and then the diffusion behavior of carbon atoms in the welded joint is theoretically analyzed. It lays a theoretical foundation for improving the quality of the welded joint of dissimilar steel.