The invention relates to a preparation method of a biological air freshener for removing methanal. The air freshener is prepared by mixing oligose, water-soluble olive oil, azelaic acid, oat polypeptide, polyquaternium, bisabolol, walnut oil, green tea essence, jojoba oil, a probiotic group, tocopherol phosphate, octanoylsalicylic acid, pyridoxine, black currant oil, fatty alcohol ether sulfate, ferment, acetic acid, aromatic and water. The air freshener is specialized in removing harmful gas such as the methanal, benzene, methylbenzene, dimethylbenzene and ammonia and various peculiar smells generated in an automobile, rapidly eliminates peculiar smell molecules, improves the quality of air, purifies the air, removes the peculiar smells, eliminates bacteria, and keeps the air persistently fresh.