The invention discloses a microstrip waveguide double-probe transition structure applicable to a millimeter-wave frequency multiplier. The structure includes an upper cavity, a lower cavity and a microstrip circuit. The lower cavity is covered with the upper cavity in a sealing way so that a rectangular waveguide cavity and a microstrip circuit shielding cavity are formed. The microstrip circuit is fixed in the microstrip-circuit shielding cavity. The microstrip circuit includes two microstrip probes, a power distribution/synthesis circuit and a dielectric substrate. The two microstrip probes and the power distribution / synthesis circuit are arranged on the same surface of the dielectric substrate. The two microstrip probes are connected with the two ends of the power distribution / synthesis circuit respectively. A distance between the center line of one microstrip probe and a rectangular-waveguide short-circuit face is 1/4 of a waveguide wavelength of an objective frequency and a distance between the center line of the other microstrip probe and the rectangular-waveguide short-circuit face is 5/4 of the waveguide wavelength of the objective frequency. The microstrip waveguide double-probe transition structure is capable of realizing same-phase superposition of an objective frequency signal and reversed-phase offset and output inhibition of a non-objective-frequency signal. The transition structure is simple in structure, convenient to manufacture and low in price.