In the generation of infrared image, a method based on signal injection is presented in order to attain a high performance-price ratio. It consists of six interacting modules. The radiance of target, background and interference arrives at the sensor after it is attenuated by the atmosphere. The radiance and attenuation of the atmosphere are computed with the LOWTRAN model. With the mathematical model of the sensor, the scaling principle of converting the radiance to the gray-scale is built. The whole 3D scene is rendered with OpenGL. By this system, the ability of recognizing and tracking target can be evaluated in the laboratory. Therefore the outfield experiments can be reduced.