The invention provides a synchronizer. The synchronizer comprises outer tooth rings, an inner tooth ring and a middle steel ring, all of which are sleeved together. The middle steel ring is arranged inside the outer tooth rings. The outer tooth rings are provided with at least one block used for limiting the inner tooth ring. The block is provided with an elastic mechanism used for pushing and ejecting the middle steel ring. The synchronizer can be combined with a tooth ring. The middle steel ring and/or the tooth ring are/is provided with a buffer mechanism used for reducing gaps. The synchronizer has following beneficial effects: by increasing axial elastic force among inner and outer tooth rings of a two-cone coupling synchronizer, a three-cone coupling synchronizer and a multi-cone synchronizer and the middle steel ring, a locking ring (the outer tooth ring, the middle steel ring and the inner tooth ring) can be smoothly released after a synchronizing process; negative phenomena such as a stepping sense, difficulty in shifting gears and gear disengage are improved; the increased axial force enables the inner tooth ring, the outer tooth ring and the middle steel ring to be relatively pressed so that knocking sound of a gearbox is reduced; the middle steel ring and/or the tooth ring and/is provided with the buffer mechanism used for reducing gaps such that impulse between the middle steel ring and the tooth ring is reduced and knocking sound generated through collision is reduced.