The crystallization characteristics of casting molding of TNT explosive was on-line studied by high-resolution X-ray computed tomography and the three-dimensional structure information and distribution rule were obtained. The results show that the rate of solidification is the highest at starting and slows down. TNT crystal is fine at first and subsequently columned and branch crystal is formed before solidification. The TNT branch crystal has obvious tropism characteristics. The shrink hole is mainly concentrated hole and small hole. The shrink holes intersect and become bigger during solidification and stable at last. They are much smaller at side and bottom and accordingly the density is higher. The volume shrink ratio is over 20% after solidification. The linear density difference between the bottom and the middle is about 1.5%-8% but up to 5%-18% between the surface and the center.