A safety injector includes: an injection module having an injection cylinder and a pullable piston rod; a needle module including a needle seat and an injection needle; the needle seat having a seat body and a buckle. In installation state, a safety sleeve covering movably around the injection cylinder between an installation position and a protection position. In the installing position, a first end portion of the safety sleeve is near the buckle and the second end portion thereof is near the handhold portion; while in the protection position, the first end portion is protruded from the injection cylinder for covering the injection needle and the second end portion is near the head portion of the injection cylinder. An inner side of the sleeve body is formed with a first and a second annular flange; and an annular groove is formed therebetween for confining the buckle.