In order to investigate the suppression effect of Al(OH)3 on the flame characteristics of methane/coal dust deflagration. The experiments of methane/coal dust deflagration under Al(OH)3 with different mass fractions were conducted by a vertical combustion pipe, and the thermal decomposition of the experimental materials were characterized by simultaneous thermal analyzer. The suppression mechanisms of Al(OH)3 on methane/coal dust deflagration were summarized. The results showed that the flame characteristic parameters of methane/coal dust deflagration decreased with the increasing of Al(OH)3 mass fraction. The flame temperature and speed were reduced significantly at 100% Al(OH)3. Al(OH)3 hindered the thermal decomposition of coal dust by making the thermal decomposition of coal dust ended in advance, reducing the weight loss rate, and decreasing the maximum heat flow rate and heat release. Al(OH)3 played the function of cooling, heat insulation, and dilution of combustible concentration, and Al(OH)3 also adsorbed the chemical molecule produced by the thermal decomposition of coal dust, thus suppressed the methane/coal dust deflagration.