according to the large aperture, long focal length zoom system design, the structure of the optical system based on the modular concept is proposed. the structure is constituted of an afocal compression telescope and a zoom system. the parts of each other are individually designed. the aberrations of them are independently. because of this, the alignment of the system and the difficulty of test are greatly reduced. it is easily replaced by changing the zoom system parts, which can achieve other different focal length and ratio. using afocal compression telescope greatly reduces the radial aperture of the zoom group, simplifies the system structure and reduces the cost. meanwhile, the variable stop is placed in the vicinity of the primary mirror. it is instead of the zoom system used in floating variable stop. in addition, the problem about large aperture zoom system pupil matching is solved perfectly. in this article, four methods of pupil matching are given and the advantages and disadvantages of them are analyzed. using this optical structure, a zoom system is designed, which is working in the visible wavelength band with variable focal length between 900mm and 4500mm, 500mm maximum aperture. the axial dimension of the system is less than 650mm. the maximum diameter of zoom system parts is less than 40 mm. moreover, the distances of the zoom group and compensating group are all less than 60 mm. besides, the motion curves of each other are given in the article. the modulation transfer function (mtf) values of the system are greater than 0.3 at 48lp/mm across different focal length and field pointing on the axis. the design results show that the imaging quality is excellent, the structure is compact, and the alignment and test are easy. the imaging requirements of zoom system are all satisfied.