The present invention discloses a piezoelectric ultrasonic microinjection device based on a flexible hinge mechanism. The device includes: a cover, a flexible hinge mechanism, a base, a screw cap and an end cap fixedly assembled together, the base being provided with a pump interface; and a micropipette fixedly mounted in the base, the screw cap and the end cap and extending outward, the micropipette being in communication with the pump interface; wherein the flexible hinge mechanism comprises a housing, a piezoelectric ceramic package module encapsulated in the housing, a central shaft fixedly mounted with the piezoelectric ceramic package module and the base, and a vibration output shaft extending from the piezoelectric ceramic package module into the central shaft, a plurality of flexible hinge beams being disposed between the central shaft and the housing.