Magnesium hydride (MgH2) was synthesized by a direct-hydriding method, and its structure was characterized by XRD, SEM and ICP. The thermal decomposition behavior of ammonium perchlorate (AP) in the presence of MgH2 was investigated by DSC. Results show that 5% MgH2 can decrease low-temperature and high-temperature thermal-decomposition peak-temperatures of AP by 35.0 oC and 44.2 oC respectively, and increase the DSC apparent heat release of AP from 0.44 kJ·g-1 to 1.20 kJ·g-1, which indicates that MgH2 has a remarkable catalytic effect on AP thermal decomposition. The catalytic effect of MgH2 was enhanced gradually with increasing of MgH2 content. However, the effect went down when the MgH2 content reached 30%. The catalytic effect of MgH2 on the thermal decomposition of AP was obviously better than that of pure magnesium powder. The mechanism of MgH2 for catalyzing the thermal decomposition of AP was also studied.