The microstructural changes of Φ20 mm×5 mm polymer bonded explosives (PBX) sample based on HMX pressed at 50-250 MPa were investigated by scanning electron microscopy(SEM). The particle size distributions of HMX were measured by laser particle size analyzer. Results show that as the porosity in samples reduces, the crystal twinning and cracking in pressed PBX increases obviously. The mean particle diameters of HMX is 34.37 μm before being pressed, and after being pressed under 50,100,250 MPa, the mean particle diameters of HMX are 31.16 μm , 27.90 μm and 26.37 μm, respectively.