Decanoic acid/porous activated carbon function material was made with microwave oven method from waste walnut shell and decanoic acid as the carrier and the phase change material respectively in this paper. Synthetic materials in different preparation stages (ie. precursor activated carbon preparation stage, porous activated carbon preparation stage and decanoic acid/porous activated carbon function material preparation stage) were measured with Fourier transform infrared spectrum. Complex network structure forming mechanism of porous activated carbon, decanoic acid embed mode, chimeric mechanism of decanoic acid and porous activated carbon were researched, respectively, in order to explain how to prepare decanoic acid/porous activated carbon function material with microwave oven method. Meanwhile, moisture property, thermal property and absorption property of this function material were tested by dynamic was used to vapor sorption, differential scanning calorimeter and environmental test chamber. The results show that the decanoic acid/porous activated carbon function material has developed pore structure and complex network composition, which adsorb the decanoic acid and formaldehyde molecules. Besides, the pore surface has some hydrophilic functional groups, which can adsorb the water molecules. The decanoic acid/porous activated carbon function material has good moisture property, thermal property and absorption property. Experiments suggest that the equilibrium moisture content can reach 0063 1~0257 g·g-1 in the relative humidity ranges from 40% to 60%. While the phase transition temperature reaches 2742~3396 ℃, the phase change enthalpy is 5214~5267 J·g-1. Besides, the adsorption efficiency of formaldehyde gas can reach 5057% after 4 h.