The present invention discloses a safety injector and a safety mechanism usable therewith. The safety injector (1) includes: a syringe (2) having a proximal end (3), a distal end (4), a chamber (5) provided between the proximal end and the distal end and defining a longitudinal axis, and a needle holding portion (6) extending from the distal end to a distal side； a plunger (7) having a distal side portion inserted into the syringe from the proximal end of the syringe, and a proximal side portion located outside the syringe； a needle (8) having a proximal side portion inserted into a needle channel of the needle holding portion from the distal side of the needle holding portion, and a distal side portion located outside the needle holding portion, wherein the needle is hollow and in communication with fluid of the chamber of the syringe； and a safety mechanism (9) at least partially surrounding the needle holding portion for installation, wherein the distal side portion of the needle is not covered by the safety mechanism in an unactuated position of the safety mechanism, and at least the terminal of the distal side of the needle is covered by the safety mechanism in an actuated position to prevent accidental contact with the terminal of the distal side of the needle. According to the present invention, when the injector is assembled, the safety mechanism is installed first and then the needle is installed so that the needle is calibrated for only one time and the needle is not interfered before the safety mechanism is actuated.本發明公開一種安全注射器和用於其的安全機構。安全注射器(1)包括：針筒(2)，具有近端(3)、遠端(4)和在近端與遠端之間且限定縱向軸線的腔室(5)，持針部(6)從遠端向遠側延伸；柱塞(7)，具有從針筒的近端插入針筒內的遠側部分和位於針筒外的近側部分；針(8)，具有從持針部的遠側插入持針部的針通道內的近側部分和位於持針部外的遠側部分，針是中空的且與針筒的腔室流體連通；和安全機構(9)，至少部分環繞持針部安裝，在安全機構的未致動位置，針的遠側部分不被安全機構覆蓋，在致動位置，針的至少遠側末端被安全機構覆蓋，以防止與針的遠側末端意外接觸。根據本發明，在組裝注射器時，能夠先安裝好安全機構，再安裝針，這樣只需校準一次針，且在安全機構被致動前不會干涉到針。
