The invention discloses an adsorbent used for efficiently separating formaldehyde from air and a preparation method thereof. An adsorbent carrier is active carbon, the surface of which is attached with one or more of -NH, -NH2 and -NH3. The precursor of -NH, -NH2 and -NH3 is one of salvolatile, ammonium bicarbonate, ammonium sulfate, ammonium nitrate, ammonium chloride, ethene diamine, hexamethylenediamine, aniline and urea. The adsorbent can absorb and separate formaldehyde in air when space velocity is 20000-80000h<-1> at room temperature. Especially when the formaldehyde and benzene seriesin the air coexist, the adsorbent not only can absorb and separate formaldehyde but also can absorb and separate the benzene series. Therefore, the adsorbent can be widely applied to the purifying treatment of air in hermetic and semi-hermetic limited spaces, such as building interiors, spacecrafts, submarines, undergrounds and the like.