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Wind barrier is an effective countermeasure to protect vehicles from the lateral wind on sea-crossing bridges. Previous studies focused on the sheltering efficiency of wind barriers and their effects on the aerodynamic/aeroelastic characteristics but ignored the analysis of intrinsic mechanism for dynamic stability, especially for flutter instability, which results only in a vague understanding of the role of wind barriers in flutter. Hence, this study paid attention to the effect mechanism of wind barriers on the flutter characteristics to generate a comprehensive understanding of the coupled relationship between wind barriers and the main structure. Wind tunnel experiments of closed box girders with different aspect ratios were first conducted to unearth the sensitivity of wind-barrier location, wind-barrier shape, wind-barrier porosity, and aspect ratios of main girders to flutter performance. The positive function of wind barriers was discovered, which was contrary to the common view that wind barriers had an adverse effect on streamlined structures. Then, the overall evolution of flutter process affected by wind barriers was investigated in detail from the flutter derivative contribution and modal participation. Simultaneously, local-region responses on the bridge surface to the installation of wind barriers were also studied by a proposed simplified local effect model based on surface fluctuating pressure. Also, the reason for the different influences of wind barriers was revealed according to the analysis from both overall and local levels. Wind barrier is an effective countermeasure to protect vehicles from the lateral wind on sea-crossing bridges. However, a modification of structural aerodynamic shape affected by wind barriers may have an adverse effect on aerodynamic stability, especially flutter instability. This study focuses on the wind-barrier effects on the flutter characteristics of closed box girders with different aspect ratios. The model test shows that there is no obvious positive or negative correspondence between wind barriers and structural flutter performance. If we can make full use of wind barriers, the flutter critical wind velocity can be improved by even 40%. Then, the flutter mechanism of wind barriers is investigated in detail based on the analysis of global derivatives, local derivatives, and unsteady pressure. Finally, the key flutter derivatives and the key function region of the deck surface are obtained, which may provide some references for the application of closed box girders in super long-span bridges. [ABSTRACT FROM AUTHOR]