The method comprises the following steps: adding a solvent into crushed Usnea diffracta Vain for extraction; and filtering, and recovering an extracting solution at reduced pressure to obtain a total extract, adding water for suspension, and using petroleum ether, ethyl acetate and n-butyl alcohol for extraction in turn, wherein the obtained ethyl acetate part comprises 8 phenolic acid compounds and 2 steroides compounds.

The method has high yield of the prepared extract, short technological flow, simple equipment and operation and low cost, and is suitable for mass preparation. The extract can inhibit activity of oral microorganisms, and can be widely used in medicines or health care products for preventing and treating oral diseases.

Usnea diffracta Vain is used as herbs in the northeastern part of China. It has the cardiotonic and diuretic effects, can relieve cough and asthma, clear and activate channels and collaterals, remove pathogenic heat to improve acuity of vision, etc. It can be used to cure conjunctivitis, corneal nebula, tinnitus, leucorrhea, rheumatic complaints, scrofula, etc.