Reduction is essential in controller design for flexible aircraft, leading to model uncertainty and modeling error. It is difficult to ensure robust stability by conventional controller. In order to resolve that problem, a robust controller, which can not only control flexible aircraft attitude but can also restrain structure vibration quickly, is designed using H_∞ mixed sensitivity control method. The controller is reduced to fourth-order and sixth-order by balanced truncation. Controlling effects of the reduction controller are studied by simulating. Simulation results show that compared with full-order controller, response performance of attitude angles using sixth-order controller isn't affected and the controlling effect for fourth-order structure transform mode of aircraft is also roughly the same. Fourth-order controller can make attitude angle of aircraft and vibration of structure transform mode stable and the stable time is similar to full-order controller.