Abstract: Wind tunnel test is one of the most important methods for vortex-induced vibration (VIV) research. Due to the limitation of the wind tunnel size, blockage-effects exist in the tests of VIV. However, the research is relatively scarce about the blockage-effects. In the numerical wind tunnel simulation, the size can be defined by users. Therefore, it can get rid of the size limitation of a real wind tunnel. Taking a flat-box-girder as the research object, numerical wind tunnel simulation was adopted to simulate the vertical VIV with blockage ratios of 1%, 2.5%, 3.89%, 5% and 8.75%. The blockage-effects of this girder were studied on its aerodynamic forces, vortex-induced lift forces and their components, vortex amplitudes, and flow fields. The results show that the aerodynamic coefficients, vortex-induced lift forces, vortex amplitudes and the flow field ranges influenced by the girder increased with the increase of the blockage ratio. The variation trend of each component of the vortex-induced lift forces is different. The linear aerodynamic damping force doing positive work decreases slightly and then increases, while the nonlinear aerodynamic damping force doing negative work continues to decrease with the increase of the blockage ratio. When the blockage ratio is less than 2.5%, the magnitudes of the changes of the above-mentioned items are almost within 5%. Finally, the amplitude correction coefficients of blockage-effects of VIV with flat-box-girder are developed by the numerical simulation results.