Bispecific antigen binding protein complex and preparation methods of bispecific antibodies

A bispecific antigen binding protein complex comprising a first polypeptide comprising a first antigen binding site at an N terminus; a second polypeptide comprising a second antigen binding site at an N terminus; and a linker connecting the first polypeptide and the second polypeptide; wherein the linker comprises a tag at one terminus thereof, and wherein the tag is connected to a C-terminus of the first polypeptide or to an N-terminus of the second polypeptide, and comprises a cleavable amino acid sequence; as well as related compositions and methods.