The invention relates to a self-locking single use syringe, comprising a plunger arm, a plunger end, a syringe body and a locking sleeve. The locking sleeve is circumferentially disposed on an outer periphery of the fixed tip of the plunger arm, so that the plunger arm can be engaged to the plunger end via the fixed tip thereof. When a user pulls back the plunger arm of the syringe, a sleeve flange configured on the locking sleeve is locked in a sleeve engaging part of the syringe body, thereby limiting the movement of the locking sleeve so that the locking sleeve is unable to be pushed forward again. The fixed tip of the plunger arm leaves from a sleeved range of the locking sleeve if the user keeps pushing the plunger arm at this moment, thereby starting to separate the plunger end from the plunger arm. Once the plunger end and the plunger arm are separated, the plunger end and the plunger arm are unable to engage with each other. Accordingly, the purpose of being unable to reuse the self-locking single-use syringe can be achieved.
