A protein complex comprising a first polypeptide comprising a first antigen-binding region; a second polypeptide comprising a second antigen-binding region; anda linker connecting the first polypeptide and the second polypeptide, wherein the first antigen-binding region is a single stranded polypeptide comprising a first light chain antigen-binding region and a first heavy chain antigen-binding region, the second antigen-binding region is a single stranded polypeptide comprising a second light chain antigen-binding region and a second heavy chain antigen-binding region, and the linker connects the C-terminal of the first polypeptide and the N-terminal of the second polypeptide, and comprises a tag including a cleavable amino acid sequence at one terminal or both terminals of the linker; as well as a bispecific antibody derived from the protein complex, and related compositions and methods.