To explore the influence of cold-pressed forming pressure on the microstructure of polymer bonded explosive (PBX), the small angle neutron scattering (SANS) signals of different pressure-formed octogen (HMX)-based PBX were measured by contrast variation SANS technique. The evolution of interfacial surface area between explosive crystal, binder and void in sample with the forming pressure was obtained by Porod’s theorem. Results show that when the forming pressure increases from 64MPa to 178 MPa, the density of sample increases from 1.55g·cm^-3 to 1.72 g·cm^-3 , and the interfacial binding rate between HMX and binder increases from 23.7% to 26.7%, and the total internal interfacial surface area per unit mass PBX, S_total decreases by 6.1% (the error of experiment is 3%), and the interfacial surface area between HMX and binder (S_HB) increases by 15.2%, and the interfacial surface area between HMX and void (S_HV) basically remains unchanged, while the interfacial surface area between binder and void (S_BV) decreases by 38.0%, indicating that the modeling powder and binder are gradually compacted during the process. When the forming pressure increases from 178 MPa to 382 MPa, the density of sample increases to 1.79 g·cm^-3, and the interfacial binding rate between HMX and binder increases to 42.3%, and S_total decreases by 11.2%, and S_HB increases by 49.0%, and S_HV decreases by 25.8%, and S_BV decreases by 45.5%, indicating that in this process, except the compaction of modeling powder and binder, a great amount of binder flows to the surface of HMX crystal. In addition, when the forming pressure increases from 64 MPa to 382 MPa, the sum of S_HB and S_HV (i.e. total internal interfacial surface area per unit mass PBX sample) remains basically unchanged, indicating that the cold-pressed process does not lead to a large number of transcrystalline breakage of HMX crystal, which is consistent with the optical microscopic results.