Abstract: A safety hypodermic syringe includes a barrel, a needle seat airtightly contained in the barrel, an elastic ring contained in the needle seat, a stopper, and a plunger coupled with the stopper. A needle is able to threadedly engage with the needle seat. After an injection treatment, the plunger can be pulled to draw the needle seat, together with the needle engaged therewith, back into the barrel, and can be removed from the stopper for recycling. With the syringe of the invention, the plunger thereof is recyclable, the needle hub thereof is able to be securely installed, the needle thereof is not able to stick out from the barrel thereof, remaining medicine therein after use is less than conventional syringes, and whether it has been used is able to be plainly indicated by a security bar.