In this paper, the finite elementmethod ( FEM) for the forward p roblem of op tical tomograpy under Robin boundary condition is described. We first analyze the theory ofOT, and then disp lay the results of experiment. From the analysis and results, we can find that they are coincident to each other. Compared with the Dirichlet boundary condition, we find that the results are a little different from bound due to the fact that the photon densities are not zero for Robin boundary condition.