A mode converter provided in the present invention includes an input multimode waveguide, an output multimode waveguide, and a first conversion waveguide, where the input multimode waveguide is configured to receive a first signal which mode is a first mode; the first conversion waveguide has an input coupling waveguide with a first effective refractive index, and has an output coupling waveguide with a second effective refractive index; the first conversion waveguide is configured to perform, by using the input coupling waveguide, evanescent wave coupling on the first signal that is in the first mode and that is transmitted in the input multimode waveguide, and couple the first signal to the second mode of the output multimode waveguide by using the output coupling waveguide, so as to obtain the first signal in the second mode; and the output multimode waveguide is configured to output the first signal in the second mode.