The molecular orbital theory was applied to study the adsorption mechanism of two oxygen molecules by coal active characteristic structure. The results show that the adsorption energy of coal active characteristic structure to two oxygen molecules is large, and oxygen molecules are easily adsorbed onto coal active characteristic structure. The adsorption position of oxygen molecules on coal active characteristic structure benzene ring is located on the top of benzene ring, while that on amino side chain is located on the top of C—N bond, and the distance between oxygen atom and nitrogen atom is relatively close. The electron transfer caused by adsorption weakens the strength of O—O bond of oxygen molecule and enlarges the bond length. The net charge of the oxygen molecules adsorbed on the side chain containing amino group is changed greatly, which proves that the side chain adsorption of oxygen is a relatively strong interaction, and the electrons it obtains are mainly provided by C14 and N21. Because the highest occupied orbital of the oxygen molecules adsorbed on the side chain obtains a large number of electrons, O—O bond is weakened, and its length is obviously increased.