By simulating the inside thermal emission of the medium and the radiation transfer process, the apparent emitting characteristics of a semi transparent medium layer with graded index (GRIN) was investigated. The pseudo source adding method was combined with the ray splitting and tracing technique in simulation.When constant and linearly distributed refractive indexes were considered for an isothermal layer, the apparent directional and hemispherical emissivities were both presented and analyzed for different parameters. In addition, the influence of the discrete sub layer number on the calculating was discussed. The results show that the graded refractive index may considerably affect the apparent emission of a semi transparent medium layer.