Recombinant Anti-Ad fiber knob x Anti-CD40 Bispecific Antibody (Tandem Fab) is designed to be expressed as three chains derived from the variable domains of anti-Ad fiber knob and anti-CD40 parental antibodies. It contains a His-tag for affinity purification. And the two Fd regions are connected by a flexible linker. This BsAb can retarget of adenoviral vectors to tumour cells. It can be used in cancer gene therapy to transfer a therapeutic gene into tumour cells aiming at selective and efficient cell killing.