Aiming at the problem of model size effects on vortex-induced vibration （VIV） of the streamlined closed-box steel girders， based on the Hongqimen Bridge （HQMB） of Nansha to Zhongshan Highway， the combination method of wind tunnel tests and computational fluid dynamics is applied to carry out the investigations. Firstly， the wind tunnel experimental investigations on VIV responses of the streamlined closed-box girder segment models with two different geometric scales were conducted. Then， the numerical simulations of the fluid-structures interaction （FSI） of the streamlined closed-box girder section for VIV responses with different geometric scales were conducted. The results show that the VIV responses of the streamlined closed-box girders have obvious model size effects， where the maximum amplitudes of the main deck model with conventional geometric scale （λL=1/60） are generally larger than that of the main deck model with large length scale （λL=1/30）. The numerical simulation results of the VIV responses of the streamlined closed-box girder with different geometric scales agree well with the wind tunnel test results， which verify the precision of the numerical method. Furthermore， the numerical results also indicate the ratio of length to width of the bridge girder model and the blocking ratio is not the key influencing factor on the size effects on VIV responses of the bridge girder model with closed-box streamlined section. Moreover， the VIV response of the main deck declines with the increase of the model scale ratio， and there are significant differences in the vortex shedding frequency distribution of the stationary girder model with different scale ratios.