Hepatitis B virus surface antigen with mutation on site 145 may cause omission in detection of hepatitis B virus with HBsAg diagnostic reagent. Monoclonal antibody against this mutation was obtained through hybridoma technique. The sub-class of the monoclonal antibody was determined to be IgG1(κ type, the titer is 1∶9×10⁴), and the monoclonal antibody has no cross-reaction to other hepatitis virus. The method of AC-ELISA which was established with monoclonal antibody and polyclonal antibody showed high analysing susceptibility for HBV mutant samples.