The X-ray interference lithography (XIL) beamline at Shanghai Synchrotron Radiation Facility was introduced. To achieve the beam deflection and optical pathway switch of the beamline, a deflecting mirror system of XIL beamline was designed. The function of the deflecting mirror system was analyzed and its adjusting mechanism, switching mechanism and cooling structure were designed respectively. The key movement of adjusting mechanism was discussed,which is the translating process of a linear motion outside the mirror chamber into a rotary motion in ultra high vacuum.Then, the relationship between repeatability and carrying capacity of the switching mechanism was analyzed, and the strength of precision screw was checked. A cooling structure was designed by integrating a mirror support mode and a cooling mode, and the cooling effect was analyzed by numerical simulation method. The simulated results show that the meridian direction slope error and the sagittal direction slope error of the deflection mirror are about 6.5 rad and 7 rad respectively. The precisions of adjusting mechanism and switching mechanism were tested by a laser interferometer and a photoelectric autocollimator, and testing results show that linear resolution capability of adjusting mechanism is up to 0.2 μm, and the repeatability of switching mechanism satisfies the technical demands.