A theoretical analysis of X-ray microscopy based on the fractional Talbot effect of phase grating and two different imaging formulas when the grating is placed in different locations is given. A cell model of water and protein is established. According to the imaging formulas and the cell model, a set of system parameters are designed and a conclusion which can be made is that mid-energy X-ray between 1 and 2 keV will be the optimal choice considering long focal depth and high image contrast. Then the imaging results with 1.5 keV X-ray is simulated and the phase distribution of the model is recoveried by the phase stepping method. Simulation results show that the phase contrast image with mid-energy X-ray is much better than the absorption contrast one in contrast and resolution under the same signal-to-noise ratio.