Abstract: As a typical blunt-body section, the vortex-induced vibration (VIV) problem of Π-shaped composite girders is particularly prominent. The vortex shedding at the corner of the main longitudinal beam is an important cause of VIV in bridges. To study the effect and mechanism of the bottom plate tilt angle of the main longitudinal beam on the VIV characteristics of the Π-shaped composite girders, static pressure tests and vibration tests on a segmental model were conducted. The VIV responses and the average and fluctuating wind pressure coefficient characteristics of the Π-shaped composite beam under different bottom plate tilt angles of the main longitudinal beam were compared and analyzed. The results show that, under the attack angles of −5° and 0°, when the tilt angle of the main longitudinal beam's bottom plate is positive, the VIV amplitude significantly decreases. Especially when the tilt angle is 20°, it completely suppresses the vertical bending and torsional VIVs under the corresponding attack angles. The change in the bottom plate tilt angle causes the airflow to reattach in the middle and front part after the separation at the leading edge of the upper surface, and this also alters the location of the maximum fluctuating pressure on the upper and lower surfaces of the main beam. The larger fluctuating pressures on the upper and lower surfaces of the main beam model and the intense pressure fluctuations at the rear of the lower surface are likely the main reasons for the large VIVs of the main beam model. Additionally, when the dominant frequency of fluctuating pressures becomes more dispersed, the VIV weakens.