Abstract: A sectional model wind tunnel test of a flat box girder in smooth flow was performed and the spanwise correlation of vortex-induced forces was investigated. The study results show that spanwise correlation of lift force becomes weakened with the decrease of vibration amplitudes at vibration amplitude-decreasing stage in lock-in range. At vibration amplitude-increasing stage in lock-in range, with the increase of vibration amplitudes, spanwise correlation of lift force becomes strengthened first and then weakened. The maximum value of spanwise correlation coefficient appears in a wind speed in vibration amplitude-increasing stage, rather than the wind speed corresponding to maximum vibration amplitude; In lock-in range, spanwise correlation of vortex-induced forces of flat box girder is dependent on both vibration amplitude and wind speed in smooth flow.