The present invention relates to a new self-locking self-destroying safety syringe, which comprises a hollow barrel, a push rod sliding within the barrel, a rubber plug in front of the push rod, and a needle seat in front of the barrel. The push rod is provided with a round platform, which is provided with an elastic ratchet that slantingly extends toward the inner wall of the barrel. The barrel is provided at the end with an increasing-diameter portion. At the transient position between the normal-diameter portion and the increasing-diameter portion of the barrel is located a slanting step, below which is a groove where the elastic ratchet can be catched when the push rod is drawn backward. The increasing-diameter portion of the barrel is provided inside with a circlip that can press against the bottom of the round platform. For a syringe with the above-mentioned structure, the push rod can be locked after injection in the following way: The push rod is provided with the elastic ratchet that extends to the inner wall of the barrel； the elastic ratchet gets stuck into the groove when the push rod is drawn backward； the slanting step above the elastic ratchet prevents the push rod from moving forward, while the circlip below the elastic ratchet presses against the bottom of the round platform of the push rod so as to prevent the push rod from moving backward； and under the action of the groove, the elastic ratchet cannot move radially either.
