In this study, the aeroelastic behavior of a shallow Π-shaped deck section of a long-span cable-supported bridge was experimentally investigated in both completed and construction-stage configurations using an elastically mounted sectional model in a wind tunnel. During the construction stage, the deck behaves as a cantilever system with reduced effective stiffness and altered boundary conditions, which may significantly influence its aeroelastic stability. The scaled model was allowed to oscillate in two degrees of freedom, namely vertical translation and torsional rotation. Wind tunnel tests were conducted within a wind velocity range of 3.27 to 7.72 m/s and under initial angles of attack from degrees. The primary objective was to evaluate the influence of structural configuration changes on the static divergence threshold and dynamic oscillation amplitudes. The results indicated that, in the construction-stage configuration, static torsional and vertical responses increased by approximately 6% and 10%, respectively, compared to the completed state. Furthermore, the dynamic oscillation amplitudes in torsion and vertical motion increased by about 8.5% and 5.5%, respectively. In certain angles of attack, the onset of instability occurred at lower wind velocities in the construction stage, indicating a shift in the aeroelastic stability boundary due to stiffness reduction and boundary condition modification. The findings demonstrate that extrapolating the aeroelastic behavior of the completed bridge to the construction stage may lead to unconservative stability assessment, and dedicated aeroelastic modeling is therefore recommended for safe design of long-span bridges during erection stages.