Abstract: This paper proposes a robust design method of disturbance observer (DOB), employs the H∞ norm to define the optimization performance evaluation function of the DOB, and transforms the low-pass filter design into an H∞ closed loop shaping problem. By appropriate processing the constraints such as relative order condition etc., the loop shaping problem with constraints can be transformed into an H∞ standard problem without constraints, and then the algorithms for standard problems can be applied to the design of the filter. Furthermore, the relationship between performance and frequency weighted functions is investigated, based on which the selection method of weighted functions is presented. The experimental results validate the optimality and systematicness of the proposed method, which is easy to implement.