Abstract: A safety syringe for intravenous injection includes: a syringe cylinder having a plurality of sleeve ribs formed in a sleeve portion eccentrically formed on the syringe cylinder, a needle device having a needle detachably coupled on an shank portion which is provided with a plurality of shank ribs on the shank portion to be engaged with the sleeve ribs in the sleeve portion, with the needle device retractable in and automatically biased in the syringe cylinder to prevent outward protruding of used needle; whereby upon external connecting and rotative coupling of the needle on the shank portion, the shank portion will be locked without being rotatable, thereby enhancing a smooth coupling of the needle on the shank portion of the needle device. The syringe cylinder is also formed with an annular extension for limiting a rearward releasing of the plunger once inserted in the cylinder.