The invention discloses a back-twist device of a synchronous belt. The back-twist mechanism comprise a back-twist driving system and a transmission system, wherein a main transmission motor drives a first belt pulley to rotate, the first belt pulley drives a second belt pulley to rotate by the synchronous belt and drives a main shaft to rotate, a third belt pulley and a fourth belt pulley which are arranged on a first group of cradle frame installation shafts synchronously rotate through the synchronous belt by a transmission belt pulley, and the fourth belt pulley on the first group of cradle frame installation shafts drives a fourth belt pulley arranged on a second group of cradle frame installation shafts to simultaneously rotate by the synchronous belt so as to achieve the purpose of back twist. The back-twist device is low in noise and is convenient to install and adjust, and the transmission speed is increased.