The solution of the diffusion equation in multiple-scattering media with photon density wave is investigated and expanded in the theoretical aspect. A second-order analytical solution of diffusion equation in semi-infinite cube space with photon density was researched. Because the flux is the general measure value, and then the relationship between 'delta''mu'_(a) and flux Jn is obtained. The relative error between second-order and first-order was presented by the light flux. It is shown that second-order solution is more precise than first-order solution in computation, and the computational precision is more increase especially around the abnormal body. The more accurate theory contributes to the image reconstruction and the quality improvement in imaging.