For the projection lithography, off-axis illumination has become one important resolution enhancement technique, which can also increase the depth of focus. It is realized by the pupil shaping unit to change the partial coherence factor. The pupil shaping unit is composed of a zoom system, diffractive optical elements (DOE) and axicons. By changing the focal length of the zoom system, a continuously adjustable diameter of the pupil is achieved. For the zoom system, its front focal plane and the back focal plane must be fixed and the telecentricity should be maintained in the process of zoom. In this paper, a zoom system with a zoom ratio of eight is designed. The system comprises only five lenses. The first lens works as the front fixed group and the last lens works as the back fixed group. The second lens is the zoom group to achieve the desired focal length, while the third and fourth lenses constitute the compensatory group to compensate the image shift. To eliminate the aberrations and obtain a compact structure, these five lenses are aspherical lenses. By using the multi-configuration design of ZEMAX software, the system is optimized to have a high imaging quality. Under the working wavelength of 193.368nm, the modulation transfer function is close to the diffraction limit. With this zoom system, the pupil shaping unit is simulated using ASAP software. The simulation results show that this zoom system can meet the requirement of pupil shaping of the lithography tool.