Abstract: The effects of Ca(OH)2 on the thermal decomposition of coal humic acid under conditions of dry mixing and wet mixing were studied by means of DTG（derivative thermogravimetry），XRD（Xray diffraction）and SEM（scanning electron microscopy）. The results show that Ca(OH)2 additive has slight effect on the evaporation of the adsorbed water and the decomposition of the function groups from humic acid below 400 °C，whether under condition of dry mixing（i.e.,without the soaking role of H2O）or of wet mixing（i.e.,with the soaking role of H2O）. The mixture of humic acid and Ca(OH)2 under condition of wet mixing，440 °C～610 °C，presents higher weightloss rate than the mixture under condition of dry mixing. When heated up to 738 °C，the maximum weightloss rate of the aromatic nucleus from the humic acid mixture with the role of H2O arrives at 0.215 8 mg/min. It is found that Ca(OH)2 plays the role of the removal of 002 diffraction peak，the CO2 absorption and the catalysis in thermal decomposition of humic acid. What’s more，for the soaking role of H2O，Ca2+ ion is easily and uniformly loaded onto the molecule of humic acid，which is prone to complex reaction. As a result，the catalyse and the thermal decomposition of humic acid are enhanced.