To investigate the effect of demineralization on Longkou oil shale, the oil shale was acid demineralised in three steps with HCl→HF→HNO₃, XRD and FT-IR was respectively used to analysis the compounds of minerals the change of chemical structure information before and after acid-treatment. The results show that oil shale mainly contains four oxygen-containing function groups, ether oxygen, carboxyl, hydroxyl and carbonyl, organic matter made by acid-treatment is really pure, the ash weight percentage is less than 1% after acid-treatment. acid-treatment has effect on the chemical structure in oil shale, hydrolysis reaction is occurred during the acid-treatment, alkyl ether and aromatic ether in the oil shale is decreased after acid-treatment; Carboxylic acid and phenolic hydroxyl are obviously increased, this is because acid-treatment breaks the ion exchange interaction between oxygen-containing groups and inorganic salts, then organic salts convert into organic acid; Hydroxyl structure both mainly contains structures of ring hydroxyl, OH-O and OH-OH before and after the acid-treatment, The acid-treatment has obvious effect on ring hydroxyl, OH-O, and has little effect on OH-OH; To the aliphatic structure, Acid-treatment makes the CH stretching vibration and CH₃ asymmetric stretching vibration convert to CH₂ asymmetric stretching vibration, the aliphatic chain is becoming longer.