A medically safety injector comprises a needle hub attached by a needle, a hollow barrel for engaging with the needle hub and guiding the needle hub to retract into the hollow barrel, and a plunger combination settled in the hollow barrel. The plunger combination comprises a retractable plunger and a hollow shank partially telescoped with the retractable plunger, wherein at least one raised portion is formed on an outer wall of the retractable plunger and at a telescoping portion for engaging at least one depression formed on an inner wall of the hollow shank so that when the plunger combination continuously receives a pushing force after the medically safety injector is used to perform the injection operation, the retractable plunger can smoothly retract into the hollow shank. Consequently, space in the hollow barrel originally occupied by the retractable plunger or hollow shank is freed for accommodating the retracted needle hub.
