The present invention relates to the technical field of diagnostic reagents, and particularly relates to a magnetic immunization in-vitro diagnostic reagent. In order to solve the technical problems of smaller capacity of an immobilized antibody, long reaction time, relatively low sensitivity and low stability of the immobilized antibody of a conventional enzyme immunoassay, the present invention provides a GP73 monoclonal antibody and a superparamagnetic nano immunization microsphere with the GP73 monoclonal antibody coupled with the surface, and a human serum or plasma GP73 antigen detection method using the superparamagnetic nano immunization microsphere by double-antibody sandwich enzyme immunoassay or chemiluminescence method. An antigen for producing the GP73 monoclonal antibody has the following amino acid sequence AAAERGAVELKK. The superparamagnetic nano immunization microsphere has the characteristics of being capable of coupling more antibodies, fast in immunoreaction speed, high in specificity, good in repeatability, low in cost, and simple in experimental condition requirements and the like.