The invention discloses a quantitative injector, belonging to the field of medical instruments. The quantitative injector comprises a shell, an injector, a push rod, a lead screw, a nut, a cavity, a circuit board, a display, a control motor, a fixing seat and a switch, wherein the display, an injection amount adjusting knob and the injector are arranged on the shell; the push rod is arranged in the injector; the cavity is formed in the shell; the bearing and the circuit board are arranged in the cavity; the lead screw is arranged on the bearing; the nut is arranged on the lead screw; the lead screw is connected with the control motor; the circuit board is connected with the display, the injection amount adjusting knob, the control motor and the switch through leads respectively; the shell is provided with a track groove at a position corresponding to the lead screw; the fixing seat is arranged on the nut; the fixing seat is positioned on the outer side of the upper surface of the shell, and is connected with the push rod; and the switch is arranged on the shell, and is connected with the control motor through a lead. The quantitative injector has the beneficial effects that: mechanical transmission is adopted, so that accurate dispensing is realized, and stable medicament flow is ensured; and full-automatic operation is adopted, so that artificial influences are reduced, and the medicament effect, medical safety and the like are ensured.