The model for infrared coded mask imaging systems was presented. It was composed with two functional components, the coded mask imaging with ideal focused lenses and the imperfection of practical lenses. The system′s point spread function(PSF) can then be represented by the diffraction pattern of the mask and the PSF of the practical lenses. The imaging results with inclined plane waves were also analyzed to achieve the variation of PSF within the view field. According to indices for mask pattern evaluation and system′s PSF assessment, mask pattern we proposed based on Dammann grating had a balanced performance for direct imaging and imaging reconstruction. Experiment shows that mask pattern for direct imaging should have more random structures, while more periodic structures in system with image reconstruction.