The invention discloses a preparation method of monostroma nitidum enhanced air filter paper. Novel air filter paper is prepared by using chopped glass fiber and glass cotton fiber different in fiber diameter, and monostroma nitidum is added, so that filtering effect is further improved. The preparation method is simple and reasonable in process, high in yield and low in production cost; the chopped glass fiber and the glass cotton fiber which are different in fiber diameter are used to prepare bilayer composite air filter paper, so that filtering efficiency is improved, wind resistance is lowered, and dust clogging capacity is improved; monostroma nitidum which is an additive is added, so that filtering effect on harmful gas like formaldehyde is further improved, and dust clogging capacity of the monostroma nitidum enhanced air filter paper is further improved.