A safety syringe with a needle sleeve is provided. The needle sleeve is put around a cap of the syringe with a shoulder portion inside the needle sleeve abutting against a rear flange of the cap, such that the cap, a hub tightly fitted in the cap, and the needle sleeve together form a needle sleeve assembly, which allows the hub to temporarily locate closely before a neck portion of a barrel of the syringe, so that the syringe may be sterilized under high temperature without stress deformation and thermal deformation. When the cap is depressed, the hub and a cannula held thereto are caused to move backward and be stably retained to the neck portion of the barrel in an airtight relation. And when the cap is pulled forward again, it brings the needle sleeve to together separate from the hub to expose the cannula for injection.