When taking the film diffractive optical elements as main lens, large aperture diffractive telescope has the characteristics such as low mass density(surface mass density can reach 01 kg/m2), loose surface shape tolerance(surface accuracy requirements are the magnitude of centimeter) and low launch costs. In this paper, the imaging theory and the configuration of diffractive telescope are discussed. Then, the calculation process of the initial structure of this optical system is derived. A prototype system with 300-mm aperture, 2-m focal length and working wavelength from 058 μm to 068 μm is designed and star image test and resolution board test are carried out. The test result of star image is close to diffraction limit, which is agree well with the design result. The work in this paper can provide theory foundation and initial model for diffractive telescope designer, and can help them to reduce design time and increase imaging quality.