Although the motivation of AlH3 enhancing combustion were recognized in many research, the promotion mechanism have been rarely explored. Herein, a previously unreported porous layer mechanism when combustion were determined in HTPB/AlH3 fuels by SEM, thermo-analysis and a new simplified calculation method, owing to rapidly released gas phase H-2 from AlH3 dehydrogenation exposing in melting layer. 5/10% 40-80 mu m and 10% 80-200 mu m AlH3-HTPB formulas show the regression rate increase by, 25.7%, 29.0% and 43.0% at Gox = 350 kg/m(2).s, while by 57.2%, 42.0% and 44.2% enhancement at Gox = 150 kg/m(2).s. The low AlH3 content (<= 10%) promotes the regression rate obviously, while excess AlH3 content (>= 20%) promotes slightly as a result of comprehensive factors combined by energy release, a certain porous layer mechanism, aggregated Al2O3 attached on the burning surface and the blocking effect of the gaseous released H-2. A new model predicting the overlapping process of AlH3 dehydrogenation and Al oxidation in air atmosphere was developed by superimposing AlH3 dehydrogenation simulation and corresponding separated Al oxidation simulation. A 1.5th Avrami-Erofeev (A-E) simulation was proposed for Al passivation weight gain between 420 and 520 K with an activation energy of 124.92 kJ/mol and the pre-exponential of 10(boolean AND)12.35.