By using the radiance transfer law, the illumination model which was widely used in computer graphics was remodeled into a simplified thermal infrared imaging model. The improvement of the model are as follows: 1) adding the surface temperature distribution and the material parameters to the geometric model, 2) introducing the self emission and the detector property into the illumination model. Based on this model, the ray tracing method was used to construct an infrared imaging system which could get the synthetic infrared images of 3D scenes from any angle of view. The infrared images were calculated for three typical 3D scenes. The calculated results were compared with the real infrared images obtained by a middle infrared band imaging camera. It shows that the infrared images obtained by thermal infrared imaging model are similar to those received by thermal infrared camera. Quantitative analysis shows that there exists difference in absolute brightness, and the reasons are analyzed.