Abstract: The analytic solution of carbon migration in dissimilar metal weldments(DMWs) had been researched by many researchers,because of the idealization of assumed conditions,it caused bigger difference between the analytic and testing result.In this paper,numerical solution of carbon diffusion was solved and the finite difference method was used,inner boundary conditions of the meshes were treated using diffusional flux and carbon's activity,the stability and the convergence equation were dissussed.The system of identification indicated that the numerical results were accordant to the experimental results.