Abstract: With application of phase change during wave propagation in radiographic testing, phase contrast imaging was developed. The difficulties in traditional radiographic testing of weak absorption materials can be overcome by this imaging method, and high contrast image can be obtained. In line phase contrast imaging method is easy to realize: phase contrast image can be acquired after coherent rays penetrated the object and propagated a certain distance in free space. In this paper, principle of in line phase contrast imaging was investigated, classification and application of the imaging technique were summarized, and the numerical simulation method for in line phase contrast imaging was analyzed.