Abstract: The optimized design method based on signal to noise ratio of the image is presented for the object of two-dimensional Gaussian distribution based on the theoretical analysis of the technology of X-ray in-line phase contrast imaging. The imaging systems under the radiation of sub-micrometer focus X-ray source, laser-based micrometer focus X-ray source and synchrotron radiation X-ray source are optimized by means of numerical simulation. The outcome shows that the method based on signal to noise ratio can increase the signal to noise ratio of the image and get more reasonable imaging distance premised by decreasing the contrast slightly when compared with the contrast method.