The catalytic activity of magnetite (Fe3O4) nanosheets, particles, and rods for the thermal decomposition of ammonium perchlorate (AP) was investigated. The TG-DTA results showed that Fe3O4 with different shapes exhibited different catalytic activity. The catalytic activity of Fe3O4 occurred mainly during the high-temperature decomposition process of AP. The Fe3O4 nanosheets with exposed high-energy {110} facets showed better catalytic activity than that of particles and rods with {111} facets. The catalytic mechanism for the thermal decomposition of AP was discussed.