To solve the problems of impure output modes and inefficient TEM transmission in the Ku-band MILO with the conventional uniform coaxial extractor, a gradual transitional coaxial extractor was proposed. The simulation results show that: the gradual transitional coaxial extractor achieves efficient microwave extraction and a single output TEM; the transitional coaxial structure is conductive to the design of supports and meets TEM transmission requirements. Compared with the traditional extractor, the gradual transitional coaxial extractor can strengthen the beam-wave interaction and accelerate the microwave start-up through enhancing the quality factor of the Ku-band MILO in right amount.