A safety syringe (10) includes a barrel (20) having a positioning groove (24), a needle holder (30) mounted in the barrel (20) and having a first internal flange (32) engaged in the positioning groove (24) of the barrel (20) and a first positioning flange (34), an interlocking seat (40) mounted in the needle holder (30) and having an elastic flap (43) and a first external flange (45), and a plunger (50) mounted in the barrel (21) and having a second external flange (52). By means of engagement between the elastic flap (43) of the interlocking seat (40) and the second external flange (52) of the plunger (50), the interlocking seat (40) can be pulled by the plunger (50) to let the first external flange (45) to be forced into engagement with the first internal flange (32) of the needle holder (30) for allowing the needle holder (30) with an attached needlestick to be pulled backwardly into the inside of the barrel (20) to assure the safety of the syringe (10) after the injection.
