The application of diffractive surface in the bifocal optical system is investigated by taking the advantage of the negative dispersion and the arbitrary phase modulation characteristics of binary elements, an analysis on the zoom equation is made and a sample design is presented. It is shown that good imaging quality can be achieved with the implementation of only four Germanium lenses. When the focal length of the system is 80 mm and the F-number is 0.8, the longitudinal aberration is less than 72 μm, the modulation transfer function (MTF) of 10 lp/mm is greater than 0.8 ；when the focal length is 160 mm and the F-number is 1.6, the longitudinal aberration is less than 35 μm, MTF of 10 lp/mm is greater than 0.7.