According to the affected images generated by the thermal imaging system which incorporates a front wire grid, with the help of classical optical theory, we analyzed the diffractive functional principle character of light wave, and also analyzed the case of non-concolorous targets, and further established the mathematic model. The results indicated that after putting in parameters of real instance and generating simulated images using MATLAB, the simulated images have good comparability with the images obtained in reality. The research achieved the expected aim of the modelling. Finally we put forward the questions and key technology pending further research.