The present invention discloses a strain of a recombinant cell line for stably expressing classical swine fever virus E2 protein, and applications of the recombinant cell line in preparation of subunit vaccines and diagnosis reagents of classical swine fever, wherein specifically the recombinant cell line is BCSFV-E2, is preserved in the China General Microbiological Culture Collection Center, and has the preservation number of CGMCC No.7719. The classical swine fever subunit vaccine prepared by using the recombinant cell line has characteristics of high safety, good immunization effect, easy mass production, less being susceptible to exogenous virus pollution or influence of antibodies, and no influence of the maternal antibody on immunization of swine, and can induce and produce high level classical swine fever virus neutralization antibodies after the swine is immunized. In addition, the present invention further discloses a method for constructing the recombinant mammalian cell line, a method for preparing the classical swine fever subunit vaccine, and applications of the antigen expressed by the recombinant cell line in preparation of classical swine fever prevention vaccines and diagnosis reagents.