Abstract: Combined with the radiosity intensity of diffusely reflecting surfaces,the improved ray tracing method is reversely employed to analyze the apparent spectral emissivity of a body of semi-transparent medium having some diffuse surfaces.The radiosity intensities of a diffusely reflecting surface are obtainde by iteratively solving the energy balance equations of all discrete zones and using the ray tracing method reversley.The apparent emissivity results obtained by this method are separately compared with the exact solutions of the semi-transparent slabs and the numerical values of disc bodies of semi-transparent medium gained by Monte-Carlo method,and the apparent emissivity of a disc body is investigated.