Wind fairings can effectively suppress the vortex-induced vibrations （VIV） of Π-shaped beams in the mountainous terrain， but their mitigation performance is highly sensitive to geometric configuration， and the underlying suppression mechanism remains unclear. This study investigates the VIV suppression mechanism of Π- shaped beams equipped with wind fairings. A surrogate model is employed to determine the optimal fairing configuration； two-dimensional CFD simulations are then conducted for the optimal wind fairing， a suboptimal wind fairing， and the original section， based on the SST k-ω turbulence model and dynamic mesh techniques. Furthermore， dynamic mode decomposition （DMD） combined with a body-fitted coordinate system is used to perform detailed comparisons of flow-field modes and vortex-flux characteristics among the three cases. The results indicate that， for Π-shaped section beams， unstable vortices above the deck and inside the lower cavity are key contributors to VIV occurrence. The original section exhibits pronounced VIV， where the 2nd mode corresponds to the vortex-shedding frequency； DMD effectively captures the higher-order modes， and the vortex flux is significant. From the 4th mode onward， the leading-edge induced vortices of the original section no longer develop into larger-scale vortices within the mid-span region， and gradually dissipate after shedding at the tail， indicating that the first 4th modes dominate the response. With the optimal wind fairing installed， the leading-edge induced vortices are almost entirely suppressed starting from the 2nd mode； unstable vortices fail to grow into large-scale structures in the mid-span region and do not shed， resulting in vortex fluxes approaching zero. Hence， compared with the original section， the installation of wind fairings fundamentally improves the flow pattern around the beam， significantly reducing the VIV amplitude and enhancing its stability.