To improve the suppression effect of NH₄H₂PO₄ powder on methane explosion, Mg(OH)₂/NH₄H₂PO₄ composite dry powder (CDP) with different quality ratios was prepared, and explosion suppression experiments were performed to determine the explosion suppression effect of the CDP. The explosion suppression mechanism of the CDP was explored by performing a thermal analysis. The results revealed that the CDP markedly reduced the K_G, T_max, and v_max of methane explosion. The different proportions of Mg(OH)₂ exhibited different degrees of improvement in the explosion suppression effect of NH₄H₂PO₄. For 8, and 10 vol% methane-air mixtures, the optimal explosion suppression ratios of the CDP were 20:80, and the minimum concentrations required for complete explosion suppression were 0.6 g/L. For 12 vol% methane-air mixtures, the optimal explosion suppression ratio of the CDP was 15:85, and the minimum concentration required for complete explosion suppression was 0.4 g/L. The introduction of 20 mass% Mg(OH)₂ could advance the start of the first and second stages of NH₄H₂PO₄ decomposition, and the mass loss temperature changed by 26.94 and 46.63 °C, respectively. The findings indicated that the decomposition products of the CDP substantially enhanced the explosion suppression effect to inhibit the methane explosion chain reaction.