Based on a series of sectional model wind tunnel tests, the effects of gap distance ratio on aerodynamic interference and vortex-induced vibration (VIV) performance of parallel twin-deck bridge were studied. The results show that the maximum amplitude of VIV and the lock-in wind speeds increase significantly compared with a single-deck bridge due to the aerodynamic interference effect; the most unfavorable range of gap distance ratio is 2~4, with the largest maximum amplitudes of VIV and a wider lock-in wind speed range; there is a phase lag in the vibration of two decks, which is affected by reduced wind speed, the amplitude of upstream deck and gap distance ratio.