Based on the optical partially coherent theory, a new integral phase contrast modulation transfer function theoretical model is established in consideration of the X-ray source area parameter. Through the simulation and calculation, a comprehensive analysis of the spatial coherence of micro-focus X-ray phase contrast imaging system is displayed. A new source size selection method and parameter optimization method are put forward under the actual experimental condition. The results show that the model containing X-ray source area factor can reflect the entire performance of optical imaging system better, and avail to the imaging parameters optimization design.