Long-span suspension bridges are prone to vibrations induced by multiple excitations, such as wind and seismic loads. This study introduces a novel strategy, damped outrigger, to enhance rotational damping of multiple modes and mitigate bridge vibrations. To verify the effectiveness of the strategy, a long-span suspension bridge with the main span of 1650 m is adopted as a case study. First, a reduced-order model of the bridge is employed to calculate the damping ratios of modes subjected to vortex-induced vibrations by modal analysis. Moreover, the variation in amplitudes of the main girder is examined under wind and seismic loads, respectively, to assess the efficiency of damped outriggers. The results show that with damping coefficients optimization aiming at the target modes, 6 out of 7 modes can attach additional damping ratio exceed 1.0%, and the corresponding amplitudes will reduce over 65% under vortex-induced forces. In addition, the seismic amplitudes could also be partly suppressed with damped outriggers. With further parameter optimization, it is anticipated that dual control of wind and seismic loads can be concurrently achieved.