Abstract: In view of the issue that the impact of different closed-loop bandwidths on the nominal control gain is neglected when dealing with under damping objects with the active disturbance rejection controller, an estimation method based on frequency domain analysis is proposed for determining nominal control gain. The frequency responses of underdamping systems with two eigenmodes are analyzed with the modal decomposition technique. For this type of underdamping object, The guideline for determining nominal control gain is investigated under the same closed-loop bandwidth. The impact of different closed-loop bandwidths on the nominal control gain estimation is studied for the same underdamping object. Simulation results demonstrate that the proposed method can achieve accurate nominal control gain estimation for these systems.