The invention discloses an automatic door lock mechanism, which is applied to a dump trailer and comprises a connecting seat, an adjusting pull rod, a hook hinged with one end of the adjusting pull rod and a drive mechanism connected with the other end of the adjusting pull rod, wherein the connecting seat is fixed on a carriage door of the dump trailer and provided with a connecting pin matched with the hook; the middle part of the hook is hinged on an upper frame of the dump trailer through a first pin shaft; the drive mechanism is arranged on the upper frame of the dump trailer; and the hook can be pushed and pulled by the adjusting pull rod under the action of the drive mechanism to rotate around the first pin shaft so that the free end of the hook hooks or is separated from the connecting pin of the connecting seat to lock or open the carriage door of the dump trailer. The automatic door opening mechanism can be simultaneously applied on two sides of a carriage, and can meet the requirement of the dump trailer for double-side dumping.