The present invention belongs to the field of bio-engineering pharmacy, protein polypeptide drugs and biomedical engineering, and relates to design and construction of a miniaturized fusion antibody providing high affinity for human epidermal growth factor receptors EGFR1 and HER2, and applications of the miniaturized fusion antibody in tumor targeting diagnosis and therapy, wherein the bispecific antibody is obtained by connecting a HER2 specific domain and a EGFR1 specific domain in series through a (G4S)3 connecting peptide and carrying out gene recombinant expression, the full-length of the fusion protein coding gene is 804 bp, 268 amino acids are coded, the molecular weight of the fusion protein is 29 kD, and the amino acid sequence is the sequence represented by SEQ ID No:1. Compared with combination of the EGFR1 and HER2 antibodies, the bispecific miniaturized antibody of the present invention provides significant in vitro and in vivo activity advantages, has advantages of low toxicity, being rapidly cleared in non-tumor sites in vivo, and the like, has high efficiency in targeting diagnosis and therapy, and can be used for tumor diagnosis and therapy.