This paper is concerned with the problem of reducing a high-order controller to a reduced-order one for flexible structures which have transverse-torsional coupled vibration mode without significant errors. Firstly, the controller is designed for the structure by using H_∝ control theory. Due to using the frequency weighting filter for compensating the unstructured uncertainty, the order of that controller leads to the high order, meanwhile, the high-order controller is not practical in implementation. Secondly, the order of the controller is reduced by using the modal truncation. This method is verified by comparing to the model reduction which has been widely used to find a reduced order controller. From the simulation and experiment results, it is confirmed that the reduced order controller obtained by the controller reduction is better than the reduced-order controller obtained by the model reduction.