A systematic investigation on the static three-component aerodynamic force coefficients of a streamlined box girder cross-section is conducted using the Computational Fluid Dynamics (CFD) approach. The Reynolds-Averaged Navier–Stokes (RANS) equations coupled with an appropriate turbulence model are employed to establish numerical simulation models of the bridge girder under various angles of attack. The flow field structures and aerodynamic characteristics around the main girder are simulated and analyzed in detail. By comparing the variations of lift, drag, and moment responses under different angles of attack, the characteristic curves of the three-component force coefficients are obtained. Furthermore, the locations of key flow separation points, the evolution of wake vortex structures, and their influences on aerodynamic force variations are examined to elucidate the underlying flow mechanisms governing the changes in aerodynamic coefficients. The results provide theoretical insights and technical support for wind-resistant design, aerodynamic optimization of streamlined box girders, and the selection of wind tunnel test parameters.