The invention discloses a method used for detecting escherichia coli O157:H7 based on an optical fiber evanescent wave biosensor. According to the method, coupling of CdTe quantum dots with escherichia coli O157:H7 polyclonal antibodies is used; optical fibers are coated with the escherichia coli O157:H7 monoclonal antibodies; an obtained optical fiber probe is inserted into a solution containing the bacteria to be detected, and immobilization of antigens on the optical fibers is realized via specific reaction of the antigens with the captured antibodies; the optical fiber probe is inserted into a solution containing detection antibodies so as to realize combination of nano quantum dots on the detection antibodies with the surfaces of the optical fibers; and then the optical fiber evanescent wave biosensor is used for detecting escherichia coli O157:H7. The method possesses significant application value in increasing detection rate of pathogenic microorganisms in food, and possesses significant importance in promoting human health and development of economic development of the state.