A safety syringe includes a barrel having a positioning groove, a needle holder mounted in the barrel and having a first internal flange engaged in the positioning groove of the barrel and a first positioning flange, an interlocking seat mounted in the needle holder and having an elastic flap and a first external flange, and a plunger mounted in the barrel and having a second external flange. By means of engagement between the elastic flap of the interlocking seat and the second external flange of the plunger, the interlocking seat can be pulled by the plunger to let the first external flange to be forced into engagement with the first internal flange of the needle holder for allowing the needle holder with an attached needlestick to be pulled backwardly into the inside of the barrel to assure the safety of the syringe after the injection.
