The aerodynamic performances of a long-span cable-stayed bridge in its completion state, which has a deck girder of open cross section, were investigated with sectional model tests in wind tunnel. Experiment results have shown that the deck of the bridge concerned suffers remarkable vortex-induced resonances within the designed wind velocity, both vertical and torsional. Parametric experiment study has shown that this vortex-induced resonance exists in an extensive scope of structural damping ratios, with amplitudes overstepping the permitted value based on recognized standard even if both the vertical and torsional damping ratios of the section model increase up to 1%. In order to suppress the vortex-induced deck oscillation, an effective vibration control scheme has been brought forward in virtue of wind tunnel tests. Model tests have shown that the adopted mitigation measure can eliminate the vortex-induced resonance effectively. Aerodynamic measure put forward will be helpful to the wind-resistance design of analogous bridges.