The thin-walled grid plate is one of the passive aerodynamic measures for suppressing vortex-induced vibration (VIV) of twin-box girder bridges. However, the effectiveness of the grid plate in aerodynamic stabilization is not fully known. In this study, we compared the flutter performance and aerostatic performance, as well as VIV performance, of a twin-box girder suspension bridge with four different grid porosities, through a series of wind-tunnel tests and three-dimensional nonlinear finite-element analyses. Results show that a thin-walled grid plate with larger grid porosity could effectively improve both the flutter performance and aerostatic performance of the bridge. This was particularly true with a plate of 100% grid porosity. Based on the 3D nonlinear bridge finite-element model, the flutter divergence mode of a 3D bridge is caused mainly by symmetrical vertical bending as grid porosity increases. Furthermore, the torsional VIV performance of a twin-box girder with smaller grid porosity is superior to those with larger grid porosity, and the application of grid plates of 30% and 50% porosity can result in VIV performance that complies fully with both the wind-resistance specification of China and the Sperling indicator.