A syringe includes a tubular syringe body, a plunger device, a needle assembly, a coupling device provided at a fluid passage opening of the syringe body for coupling the needle assembly with the syringe body, a locking member, and a puncture proof protective sleeve selectively and slidably mounted on the syringe body to move from an injection position to a protective position in one-way manner, wherein in the injection position, the puncture proof protective sleeve is slid to receive the syringe body and to expose the needle assembly for injection use, wherein in the protective position, the puncture proof protective sleeve is slid to receive the needle assembly so as to prevent the needle assembly from exposing to an exterior of the puncture proof protective sleeve, wherein the locking member is arranged to lock the puncture proof protective sleeve at the protective position.
