The present invention discloses a locking device, a power assembly, a power transmission system and a vehicle. The locking device includes: a first flange, the first flange being adapted to be fixed on the first shaft; a second flange, the second flange being adapted to be fixed on the second shaft; a synchronizing ring, the synchronizing ring being normally connected to the first flange to be adapted to rotate synchronously with the first flange, and the synchronizing ring being slidable relative to the first flange; and a driving component, the driving component selectively pushing the synchronizing ring to slide from an unlocked position to a locked position in an axial direction of the first flange. When the synchronizing ring is in the locked position, the synchronizing ring is connected to the second flange be adapted to rotate the second flange synchronously with the first flange. When the synchronizing ring is in the unlocked position, the synchronizing ring is separated from the second flange. The locking device according to embodiments of the present invention can realize the single-way locking function, and is simple in structure.