The concept and form of a space-based laser local oscillator in an infrared radio telescope with a 6.5 m diffractive synthetic aperture are proposed to satisfy astronomical observation and deep space exploration requirements. The form of laser local oscillator array detector is established, and the structure of the synthetic aperture infrared radio telescope based on a diffractive optical system is designed. The telescope uses the signal processing method for aperture transition compensation to expand the spectral range, exhibiting low complexity, small size, and light weight in the optical system. The main parameters of the system and the imaging simulation results are presented. When the central wavelength is 1.55 μm, the angular resolution is approximately 0.24 μrad, the maximum unambiguous field of view angle is approximately 1.55 mrad, and the spectral range is 0.2 μm. Its detection sensitivity is two times higher than that of the traditional 6.5-m-aperture telescope, and the observable limit magnitude is higher than 21.