A syringe (1) is applied to connect with a vial (2) and a needle (3), and the syringe (1) includes an outer protecting housing (10), a connecting ring (20), a vial housing (30), an inner tube (40), a bidirectional guiding tube device (50), a screwing sleeve (60), a ratchet (70), a spiral rotating rod (80) and an injection rod unit (90). The connecting ring (20) is mounted on a front end of the outer protecting housing (10), and the vial housing (30) is connected with the connecting ring (20) for assembling a vial (2). The inner tube (40), the bidirectional guiding tube device (50), the screwing sleeve (60), the ratchet (70), the spiral rotating rod (80) and the injection rod unit (90) are mounted in the outer protecting housing (10). The injection rod unit (90) is rapidly pulled backward and then is pressed to move forward for injecting. A large pitch stroke of the bidirectional guiding tube device (50) cooperates with a small pitch stroke of the screwing sleeve (60) with the spiral rotating rod to transform a pressing force into a rotating force. The rotating force of a large stroke is transformed into an injecting force of a small stroke to achieve the purpose of dose controlling, and the syringe (1) repeats opening and closing in an operating process to save labor.
