The present invention relates to a volume-adjustable micro-injection device. The device includes a base structure having a syringe positioning structure and a grip, in which the syringe positioning structure can flexibly accommodate injection syringes with different volumes; a holding structure capable of flexibly adjusting an injection angle of syringe content for easier operation; a qualitative controller capable of accurately controlling injection volume; a pressure pushing structure to hold and push a plunger; an injection controller interlinked with the qualitative controller and the pressure pushing structure; and an eject structure facilitating simple operation and easy replacement of injection syringes. In contrast to conventional structures, the present invention provides advantages that control injection volume more accurately, address better injection angle control, allow for the syringe contents to be free from air exposure, require no special syringes, and allow for single-handed replacement of the injection syringe.