Automatically retractable medically safety injector and plunger combination thereof

A medically safety injector, comprising:

• 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 after the medically safety injector is used to perform an injection operation;
• and
• a collapsable plunger combination settled in the hollow barrel, comprising a retractable plunger near the needle hub 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 at a telescoping portion, where the retractable plunger and the hollow shank are partially telescoped, 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 raised portion detaches from the depression to force the retractable plunger smoothly retract into the hollow shank such that space in the hollow barrel can be spared for accommodating the retracted needle hub.