In order to get the initial structure of the bifocal optical system quickly, a bifocal and dual-field optical system was designed according to the theory of paraxial optics. The initial position of the optical elements near the axis of the optical system was solved by Gauss optics theory and the principle of object and image exchange. The standard lens was inserted into the solution position by grouping. The lens spacing was optimized by gradually increasing the focal length of the elements near the axis, so that the focal length of the inserted lens set approached the theoretical calculated value of the focal length of the element near the axis. Then the method was used to completes the optimal design of each lens set. An optical system with a focal length of 40/120 mm and a field of view of 8.6°/2.9° was designed by this method. All the lenses were spherical. At Nyquist frequency of 100 lp/mm, the modulation transfer function at 120 mm focal length was 0.55, close to the diffraction limit. The modulation transfer function at a focal length of 40 mm was 0.4. The design results show that this method is suitable for dual-field optical system, and the initial structure of optical system can be obtained quickly, which greatly reduces the difficulty of design.