The radiative transfer solution method of a two-dimensional graded index semitransparent medium was developed. The apparent directional emissivity of the medium with nonlinear refractive index distribution was also analyzed. The results show that the numerical technique of this method has a high numerical precision by which the maximum relative error of apparent directional emissivity is less than 1.6%. Results also show that apparent emission of the medium has an obvious directional pattern. This phenomenon is named as a quasi-coherent thermal emission.