To study the microscale combustion characteristics of ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) composite propellant particle, a 2D steady state combustion model of a periodic sandwich unit was established based on gas-solid coupling, using simplified two-step global chemical reaction kinetic mechanism. The microscale combustion characteristics of AP/HTPB were numerically analyzed by FLUENT software. Results show that under the condition of volume fraction of AP as 0.75, at the low pressure of < 0.4 MPa, the overall flame of AP/HTPB combustion is a premixed combustion and the burning rate of AP is lower than that of HTPB. With increasing the pressure, when the pressure is more than 2.5 MPa, the flame reveals in a diffusion structure and the burning rate of AP is higher than that of HTPB. The higher the pressure, the stronger the thermal feedback from gas phase to solid phase, and the higher the temperature and burning rate on coupled surface. The volume release rate in the gas phase increases with increasing the pressure, the exothermic region contracts and the two connected exothermic core regions split to two independent core regions.When the combustion pressure is constant (2.5 MPa) and the volume fraction of AP changes from 0.7 to 0.95, the smaller the AP content, the relatively larger the binder HTPB width in a periodic sandwich unit, the flame surface tends to split into two narrow and long strip flame surface, and the temperature increases with splitting.