Abstract: An ultra-high zoom optical system with the focal length of 8 mm~2 400 mm was designed for visible light band imaging. Using multi-component fully dynamic zoom structure, it does not increase the size of system on the premise of high variable ratio.Based on the ordinary diffractive lenses, the imaging characteristics and dispersion of harmonic diffractive element were analyzed, as well as the chromatic aberration and secondary spectrum introduced by reducing the long focal length. The new aspherical equations and characteristics were given, which could solve the problem of complex selection of the number of ordinary aspherical equations.On the basis of the above theory, the system was simulated by Zemax optical design software, and 4 harmonic diffraction surfaces and 4 new aspheric surfaces were introduced. The design results show that, at the Nyquist frequency of 50 lp/mm, the modulation transfer function curves are all above 0.5. The imaging quality is good, and it can be widely used in military and aerospace fields.