A medical safety injector includes: a needle hub connected with a needle； a hollow barrel for retaining the needle hub and, upon completion of an injection, guiding the needle hub to retract into the hollow barrel； and a plunger combination disposed within the hollow barrel. The plunger combination includes a retractable plunger and a hollow shank partially muff-coupled with the retractable plunger. The retractable plunger has an exterior sidewall formed with at least one protrusion in a muff-coupled region with the hollow shank, for being correspondingly snap-fitted with at least one recess formed on an interior sidewall of the hollow shank. If a user continues to apply a force to the plunger combination after the medical safety injector accomplishes an injection, the retractable plunger will retract smoothly into the hollow shank to free space in the hollow barrel for receiving the needle hub, thereby improving single-handed operability of the injector.
