Solid propellants optimization significantly enhances combustion efficiency of the rocket engine. Aluminum （Al） particles are widely used as metallic additives due to the high reactivity and energy density. A two-dimensional homogenized steady-state combustion model with a sandwich structure was developed for AP/HTPB/Al propellants based on a five-step gas-phase reaction mechanism. The reliability of the model was validated by comparing simulation and experimental data. The results show that as the pressure increases from 0.2 MPa to 6.5 MPa transits flame structures from premixed to diffusion combustion， while the peak area of heat release gradually approaches the burn surface. At constant pressure， the burn rates rise significantly with higher Al content， however， the pressure index first decreases and then increases with the increase of Al content. This indicates that there is a trade-off between high burn rates and flame stability.