The invention discloses a synchronizer. The synchronizer is characterized by comprising a main shaft, an output shaft sleeve, a transition piece and an elastic piece, wherein a sliding groove is formed in the main shaft, sliding blocks are connected to the inner part of the sliding groove in a sliding manner, a lug is arranged on the sliding blocks, a radial pressure spring is also mounted on the main shaft, and a ring groove is formed in the output shaft sleeve; a slippage block is connected to the lug in a sliding manner, and the end part of the transition piece is hinged to the slippage block; a first limiting part is arranged on the transition piece, and a second limiting part is arranged on the outside wall of the lug; the elastic piece provides an elastic bearing force in the direction of the output shaft sleeve for the transition piece, and enables an appointed angle a to be formed between the length direction of the transition piece and the sliding direction of the sliding blocks; the output shaft sleeve can promote the sliding blocks to slide, and when the first limiting part and the second limiting part are matched with each other, the sliding blocks stop moving; and at the moment, a final angle b is formed between the length direction of the transition piece and the slippage direction of the sliding blocks, wherein the appointed angle a is greater than the final angle b, so that the purpose of high whole use smoothness is achieved.