An automatic syringe, comprising: a housing; a needle guard cover; a push rod; a prompting cylinder; an inner tube; a release ring; a driving elastic member; and a resetting elastic member; wherein a syringe fixing structure for fixing the syringe is arranged in the housing; an opening for a needle to extend out is arranged at a front end of the housing; the release ring and the needle guard cover are sequentially and slidably arranged on an inner side of the housing from a rear end to the front end of the housing; the needle guard cover is configured to extend out of the front end of the housing and slide towards the rear end relative to the housing to push the release ring; the resetting elastic member is configured to act on the release ring and drive the release ring to push the needle guard cover to slide towards the front end of the housing; the housing is provided with a needle-guard-cover limiting structure for limiting a forward sliding distance of the needle guard cover relative to the housing; the push rod is slidable in the housing along an axial direction; a front end of the push rod is configured to push a piston in the syringe; the driving elastic member is configured to act on the push rod and drive the push rod to slide towards a front end of the syringe; a push-rod limiting structure is arranged on an outer circumference of the push rod; the prompting cylinder is slidably sleeved on the outer circumference of the push rod along the axial direction; the driving elastic member is configured to act on the prompting cylinder and drive the prompting cylinder to slide towards a rear end of the housing; the prompting cylinder is provided with a push-rod-limiting cooperating structure; a prompting-cylinder limiting structure is arranged between the prompting cylinder and the inner tube; the inner tube is fixed inside the housing and sleeved on an outer circumference of the prompting cylinder; the release ring is slidably sleeved on an outer circumference of the inner tube; the inner tube is provided with a first clip capable of elastically moving along a radial direction, and the first clip is deviated radially outward under its own elastic force; and the first clip is configured to be deviated radially inward under limiting from an inner wall of the release ring, and be snap-fitted and position-limited with the push-rod limiting structure through the push-rod-limiting cooperating structure; under support of an outer wall of the push rod, the prompting-cylinder limiting structure is configured to limit sliding of the prompting cylinder relative to the inner tube; and after the push rod has slid towards the front end for a preset distance, the prompting-cylinder limiting structure is no longer supported by the push rod. a housing; a needle guard cover; a push rod; a prompting cylinder; an inner tube; a release ring; a driving elastic member; and a resetting elastic member; wherein a syringe fixing structure for fixing the syringe is arranged in the housing; an opening for a needle to extend out is arranged at a front end of the housing; the release ring and the needle guard cover are sequentially and slidably arranged on an inner side of the housing from a rear end to the front end of the housing; the needle guard cover is configured to extend out of the front end of the housing and slide towards the rear end relative to the housing to push the release ring; the resetting elastic member is configured to act on the release ring and drive the release ring to push the needle guard cover to slide towards the front end of the housing; the housing is provided with a needle-guard-cover limiting structure for limiting a forward sliding distance of the needle guard cover relative to the housing; the push rod is slidable in the housing along an axial direction; a front end of the push rod is configured to push a piston in the syringe; the driving elastic member is configured to act on the push rod and drive the push rod to slide towards a front end of the syringe; a push-rod limiting structure is arranged on an outer circumference of the push rod; the prompting cylinder is slidably sleeved on the outer circumference of the push rod along the axial direction; the driving elastic member is configured to act on the prompting cylinder and drive the prompting cylinder to slide towards a rear end of the housing; the prompting cylinder is provided with a push-rod-limiting cooperating structure; a prompting-cylinder limiting structure is arranged between the prompting cylinder and the inner tube; the inner tube is fixed inside the housing and sleeved on an outer circumference of the prompting cylinder; the release ring is slidably sleeved on an outer circumference of the inner tube; the inner tube is provided with a first clip capable of elastically moving along a radial direction, and the first clip is deviated radially outward under its own elastic force; and the first clip is configured to be deviated radially inward under limiting from an inner wall of the release ring, and be snap-fitted and position-limited with the push-rod limiting structure through the push-rod-limiting cooperating structure; under support of an outer wall of the push rod, the prompting-cylinder limiting structure is configured to limit sliding of the prompting cylinder relative to the inner tube; and after the push rod has slid towards the front end for a preset distance, the prompting-cylinder limiting structure is no longer supported by the push rod.