The analysis resolution of the diffusion equation under the condition, which is infinite cube, was predigested by using matrix transform method. And the simulation velocity was improved much. Using the presented typical optical parameter of human being＇s forearm tissue, the simulation result of the analysis resolution of the diffusion equation was shown. The influences of the medium＇s basic optical parameter on the photon density wave were analyzed. These results validate the theory that have been provided, and if was found that the analysis resolution is not applicable for the conditions that diffusion coefficient is large.