The invention discloses a follow-up device capable of driving a semitrailer
carriage to move longitudinally and a semitrailer applying the follow-up device. The follow-up device is located between a
tractor and a semitrailer
carriage and comprises a
saddle, a slider-
crank mechanism, a generating mechanism and a driving mechanism. When the semitrailer turns, as an included angle is formed between a
tractor cab and the axis of the semitrailer
carriage, the driving mechanism drives the
crank sliding block mechanism to drive a multiplication planetary gear to longitudinally move backwards in a tooth groove formed by a fixed rack and a movable rack, the fixed rack drives the multiplication planetary gear to rotate, and therefore the multiplication planetary gear drives a
saddle bottom plate to longitudinally move backwards; the follow-up backward movement adjustment of the semitrailer carriage is achieved, and the gap between the
tractor cab and the semitrailer carriage of the semitrailer is enlarged; after turning is finished and the relative rotation returns to the straight running, the driving mechanism drives the generating mechanism to reset, the
saddle longitudinally moves forwards, the gap between the tractor cab and the semitrailer carriage is reduced again, and the whole vehicle recovers to the wind resistance reducing state.