Abstract: This paper analyzes the principle and method of X-ray phase contrast imaging technique used to characterize the microstructure of polymer foam. The analytical result shows that the image contrast of low Z polymer foam can be considerably improved with X-ray phase contrast tomography. The three dimensional microstructure of polymer foam is observing combined phase contrast tomography with computed tomography, and the characterization method of foam density distribution is introduced by calculating the centroid of framework “particle”. The experimental result indicates that, the three dimensional density distribution of foams can be completely characterized through phase contrast computed tomography technique and the statistical method of framework “particle” centroid.