Abstract: Disclosed is a bispecific antibody with dual Her2 binding sites, comprising (a) an anti-CD3 antigen-binding fragment Fab, having a light chain variable region VL, a light chain constant region CL, a heavy chain variable region VH and a heavy chain constant region CH1; (b) an anti-Her2 single domain antigen-binding fragment VHH1, linked to the C-terminus of the CL of the Fab and can bind to a first Her2 epitope; and (c) an anti-Her2 single domain antigen-binding fragment VHH2, linked to the C-terminus of CH1 of the Fab and can bind to a second Her2 epitope; the first Her2 epitope and the second Her2 epitope are non-overlapping epitopes of Her2. The bispecific antibody has a killing effect on Her2 tumors with an IHC score of +1, and is effective on trastuzumab-resistant tumors.