The optimization problem of the disturbance observation performance is investigated for the third-order nonlinear extended state observer(ESO). Firstly, the frozen-coefficient method is employed to fix the nonlinear coefficient related to the observer state, and the bandwidth is extended by means of the pole assignment method in linear system theory. Then, the variation of the assigned poles with the nonlinear coefficient is analyzed. Consequently, an optimized parameter configuration method is proposed to minimize the variation of the disturbance observation bandwidth with observer state. Finally, the simulation comparison is conducted to show the effectiveness and superiority of the proposed method.