To realize the broadband imaging of large aperture diffractive optical systems, we analyze the problem of increasing relay lens diameter due to the increased bandwidth of traditional diffractive lenses based on the Schupmann structure optical path model. We propose using a harmonic diffractive lens as the primary mirror to construct a large aperture broadband diffractive optical system. A large aperture optical system with an aperture of 10 m and a spectrum covering 400?900 nm is designed, and the aperture of the relay lens is reduced by 2.4 m compared with the traditional design. To verify the design method, an imaging optical system with an aperture of 80 mm and a spectrum of 400?900 nm is designed, and imaging experiments are carried out on the system. By checking the resolution target image with no chromatic aberration, the broadband imaging design method based on a harmonic diffractive lens as the primary mirror is verified, which provides an idea for designing a large aperture diffractive optical system.