Abstract: The wind tunnel tests of sectional models for twin parallel bluff-box girders were performed firstly. Vortex-induced vibration amplitude and wind velocity locking region of twin parallel box girders in 10 different spaces were investigated and compared with those of single box girders. The space ratio of D/B is from 0.2 to 4.0, where D is net space between twin box girders and B is width of a single box girder. Then the influencing mechanism of vortex-induced vibration was studied based on flow fields attained from CFD numerical calculation. The study results show that the influence of a leeward box girder on the vortex-induced vibration of a forward box girder is shown as an amplification effect when 0.2 ≤ D/B ≤ 2.0 and can be ignored when D/B ≥ 2.5. The influence of a forward box girder on the vortex-induced vibration of a leeward box girder is shown as a notably amplification effect. The amplification effect is the most significant when D/B=0.8 and cannot be neglected even when D/B=4.0.