A rapid and automatic syringe comprises a rear casing (5), a front casing (6) connected to the rear casing (5), a sleeve (3) fitted in the rear casing (5), a support (1) fitted in the front casing (6) and abutting a front end surface of the sleeve (3), a cartridge and an injection needle mounted in the support (1), a plunger (2) mounted in the sleeve (3) capable of pushing the cartridge forward, and a trigger member (4) fit as a sleeve between the front casing (6) and the support (1) capable of moving a set distance backward, so as to drive the plunger (2) to move backward. A snap-engaging mechanism capable of retracting inwardly is provided at the rear end of the plunger (2), and an engagement block in an annular groove of the sleeve (3) is capable of being caught in the snap-engaging mechanism. A groove having an inner diameter smaller than the outer diameter of the snap-engaging mechanism is arranged in the rear end surface of the rear casing (5) and is used to retract the snap-engaging mechanism. A driving spring (7) in an initially compressed state is fitted at the periphery of the plunger (2). The front end of the driving spring (7) abuts a protruding ring in the middle of the plunger (2) and the rear end of the driving spring (7) abuts the engagement block. The driving spring (7) releases when the outer diameter of the snap-engaging mechanism is smaller than the inner diameter of the annular groove, such that the plunger (2) is driven to move forward. The rapid and automatic syringe effectively resolves an issue in which it is inconvenient to deliver liquid medicine using a syringe.
