Abstract: Diffraction enhanced imaging (DEI) is one of the most advanced technologies that differs from traditional radiography in generating image contrast and will play an important role in the field of X-ray medical diagnostic imaging. Compared with conventional absorbing imaging, DEI can greatly improve the detail and get higher contrast and sensitivity with the same radiation dose, especially for objects composed of low atomic number elements such as C, H, O and N. It makes the DEI have a prominent potential value in medicine. Classical X-ray imaging only produces absorbing contrast which is based on absorbing coefficient difference for various materials. But DEI has multiple contrast, such as absorbing contrast, extinction contrast and refraction contrast. Due to the analyzer crystal Si(111) converting refraction angle into changing of X-ray intensity distribution, refraction contrast is better for the image quality fundamentally. Spatial resolution is an extremely important parameter that characterizes overall performance of image system, but there is no theory to objectively evaluate spatial resolution of DEI until now, so it’s necessary to present a method to evaluate the spatial resolution of DEI prototype. Modulate transfer function (MTF) is an objective method for identifying the spatial resolution of imaging system. The theory model of spatial resolution of DEI system was established by calculating the MTF and the response of each factor to the spatial resolution was analyzed and explained. The theory model takes into account the inherent influence of the geometric factors by geometry optic approximation (including non-linear conversion when analyzer crystal converts refraction angle into X-ray intensity changing and limit of maximum diffraction angle). The image degenerates in quality during acquisition because of CCD pixel size and scintillator’s fluorescence dispersion effect caused by comprehensive influence of transportation of fluorescent photon and secondary electron. The geometric distortion is the primary factor contributing to image degeneration, and fluorescence dispersion effect also has some impacts on image spatial resolution. The analysis of the MTF provides the theoretical basis for the physical design and equipment selection of the DEI experiment platform.