In order to achieve a better control effect, the fractional-order was used to describe the damping force, which was supposed to be an α-order derivative of the displacement vector, and a variational-order controller design method was developed. The detailed process is as follows: First, it is assumed that the damping force in the original equation of motion is a β-order derivative of the displacement vector and the controller is designed according to this assumption. Second, considering the equation of motion that includes an α-order derivative as the control object, a control strategy based on systemic output is applied to ensure that the designed controller can be used for the original model. Third, the optimal parameter β is determined with the search method according to the third generation of the benchmark building control performance index. In this study, there was a damping effect when the fractional-order ranged from 0 to 2, and the search scope was between 0 to 2. The simulation results show that optimal parameters can be found to obtain a better control effect when β is different from α. Meanwhile, the selection of the optimal β varied with different performance indices. Finally, a simulation example verified the feasibility of the proposed control strategy.