Based on thermodynamics and energy conservation law, an elastic body physical model is established. A physical equation of infrared radiation temperature and radiant exitance is achieved. With finite element method(FEM) the model can be used to analyzing infrared radiant energy of a three-point bending beam of elastic-photo material subjected to external loading in room-temperature. It shows that results calculated are highly coincidence with those of experiments. The model and relative theories are rational in quantitative analysis. A quantitative method is shown to analyse and reveal mechanism of infrared radiation of solid materials by computer.