An automatic uncoupling mechanism for couplers comprises a coupler knuckle spindle and a driving unit comprising a cylinder body hinged to a coupler body and a telescopic member; and further comprises a first rotating member, a boss and a boss stopper, wherein the first rotating member comprises a crank hinged to the telescopic member and a rotating part being sheathed on the coupler knuckle spindle. The driving unit unidirectionally drives the telescopic member so that the rotating part drives the coupler knuckle spindle to unidirectionally rotate by the contact of the boss with the boss stopper, realizing coupler uncoupling; and, after the driving unit drives the telescopic member to return to its position, the rotation of the coupler knuckle spindle for achieving coupler coupling is not limited by the rotating part. The present application may reduce the lateral force of the coupler knuckle to the driving unit during the uncoupling process of the coupler, and may cause the spring to drive the coupler knuckle to rotate rapidly and lock the coupler during the coupler coupling process.