To study the energy releasing characteristics of aluminum powder in HMX based explosives, the quasi-static pressure of HMX/HTPB 88/12 and HMX/Al/HTPB 53/35/12 explosives containing aluminum powder with granularity of 13 μm and 130 μm respectively after explosion in an enclosed space was measured by pressure sensor. Results show that the quasi-static pressure after explosion of HMX /HTPB 88/12 and HMX/Al/HTPB 53/35/12 explosives in an enclosed condition can be produced. The quasi-static pressure of the HMX/Al/HTPB 53/35/12 explosive is 1.24 times greater than that of the HMX/HTPB 88/12 explosive. The quasi-static pressure of HMX/Al/HTPB 53/35/12 explosives containing aluminum powder with granularity of 13 μm and 130 μm respectively, produced by explosion in an enclosed space is 378 kPa and 347 kPa, respectively, meaning that under implosion condition when the content of aluminum powder is 35%, compared with the explosive with the large particle aluminum powers, the explosive with small particle aluminum powder can release more energy and improve the quasi-static pressure in an enclosed space.