A syringe is applied to connect with a vial and a needle, and the syringe includes an outer protecting housing, a connecting ring, a vial housing, an inner tube, a bidirectional guiding tube device, a screwing sleeve, a ratchet, a spiral rotating rod and an injection rod unit. The connecting ring is mounted on a front end of the outer protecting housing, and the vial housing is connected with the connecting ring for assembling a vial. The inner tube, the bidirectional guiding tube device, the screwing sleeve, the ratchet, the spiral rotating rod and the injection rod unit are mounted in the outer protecting housing. The injection rod unit is rapidly pulled backward and then is pressed to move forward for injecting.
