Abstract: The Π-shaped composite girder is a common cross-section type for long-span cable-stayed bridges, but its blunt-body profile makes it susceptible to vortex-induced vibration (VIV), threatening the bridge’s durability and structural safety. Sectional-model wind-tunnel tests were carried out to systematically examine how a lower central stabilizer (LCS) affects the girder’s VIV behavior and to identify optimal vibration-mitigation parameters. Results show that both the height and porosity of the LCS markedly influence VIV. When the dimensionless LCS height is between 1.33 and 1.67, the control effect is the greatest, reducing the vibration amplitude by 31.72%; an LCS that is too low or too high delivers little benefit. An LCS porosity of 16.8 % yields the best mitigation, lowering the amplitude by 42.0 %, whereas higher porosities actually intensify the vibration response.