Abstract: Taking an arch bridge as the example, the aerodynamic interference between two rectangular arch ribs in tandem arrangement and the effect on the aerodynamic coefficients of the cross sections are studied by numerical simulations. After checking the reliability of the numerical models, the flow fields are simulated around the arch ribs with different aspect ratios, spacing ratios, and angles of attack. The variations of the aerodynamic coefficients are explained based on pressure contours and turbulent kinetic energy contours. The relationships are discussed between the vortex shedding frequencies with different aspect ratios and the structural modal frequencies. The differences of the lift time series are also analyzed between the two arch ribs, which may increase the whole torsional moment. The results show that the aerodynamic interference is significant between the two arch ribs in tandem arrangement. In the wake of the upstream one, the drag coefficient of the downstream one largely decreases, which is closely related to the form, to the vorticity, and to the motion trail of the vortices. The contrast of the lift time series of the two cross sections shows the differences in amplitude and phase. As a result, the whole torsional moment increases, which is significantly enhanced with the increase of aspect ratio or of spacing ratio, while slightly weakened with the increase of attack angle. The vortex shedding frequency increases first and then decreases with the increase of aspect ratio, while it is less affected by the spacing ratio and by the attack angle. The variation of the vortex shedding frequency versus the aspect ratio is fitted by a polynomial and the structural modal frequencies are compared, which provides a reference for the aerodynamic configuration design of the arch.