Provided is a novel and artificially designed triabody comprising three polypeptide chains, wherein the first polypeptide chain comprises a first heavy chain variable domain, the second polypeptide chain comprises a first light chain variable domain, the first heavy chain variable domain is paired with the first light chain variable domain to form a first antigen binding site； and the third polypeptide chain comprises a second single domain antigen binding site and a third single domain antigen binding site. Provided are also a polynucleotide encoding the triabody, a vector comprising the polynucleotide, host cells comprising the polynucleotide or the vector, an immunoconjugate comprising the triabody, a pharmaceutical composition comprising the triabody or the immunoconjugate thereof, and use of the triabody in immunotherapy, prevention and/or diagnosis of a disease.
