To improve the combustion and ignition performances of ammonium perchlorate (AP)-based composite solid propellants, the manganese alginate film was prepared by an ion exchange method, and the nano-Mn₃O₄ composite catalyst was obtained after calcinations of the film. The effect of nano-Mn₃O₄ composite catalyst on the thermal decomposition performances of AP was studied. The morphology and structure of prepared nano-Mn₃O₄ composite catalyst were characterized by scanning electron microscopy(SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectrometer (XPS) and X-ray diffractometer(XRD). Results show that after the exchange of manganese ion, sodium alginate becomes manganese alginate, formed film surface is smooth and dense. After calcinations at 400 ℃, in situ-grown nano-Mn₃O₄ particle is loaded onto the carbonized alginate skeleton. The catalytic effect on AP enhances with increasing the content of nano-Mn₃O₄ composite catalyst and the exothermic rate also increases significantly. Compared with pure AP, the decomposition temperature of AP decreases by 89.1 ℃ when the content of nano-Mn₃O₄ composite catalyst is 3%.