The composite energetic material nPS/NaClO₄ was prepared by using nano porous silicon (nPS) as reducing agent and sodium perchlorate (NaClO₄) as oxidizer. And its thermal decomposition was analyzed by differential scanning calorimetry (DSC)-thermogravimetric (TG) analysis in a wide temperature range from 25 ℃ to 1200 ℃. To better understand the properties of nPS/NaClO₄, the thermal decomposition of nPS, NaClO₄, Si/NaClO₄, nPS/NaCl were analyzed, respectively. Results show that the Si—H bonds was broken around 400.0 ℃ in the oxygen atmosphere, and 820.0 ℃ in the argon atmosphere. The exothermic decomposition peak for NaClO₄ was 587.8 ℃ with the total mass loss of 68.31%. The exothermic decomposition peak for nPS/NaCl composite material was 883.3 ℃ with heat release of 567.0 J·g^-1. The heat releasing for nPS/NaClO₄ reached 359.5 J·g^-1, 15.3 J·g^-1 more than that of Si/NaClO₄ under the same condition, which may be due to the existence of Si—H bonds. Based on the analysis of thermal test results, thermal decomposition mechanism for nPS/NaClO₄ composite energetic material may be as follows: the cleavage of the Si—H bonds is in advance for the existence of O₂, which participates in the exothermic reaction. The remaining Si—H bonds after 800 ℃ react with NaCl and generate Si. The XRD spectra of solid combustion product of nPS/NaClO₄ allow a convincing check of the validity of the mechanistic assumptions.