A 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 plunger 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 plunger so that when the plunger combination continuously receives a pushing force after the safety injector is used to perform the injection operation, the retractable plunger can smoothly retract into the hollow plunger. Consequently, a space in the hollow barrel can be spared for accommodating the retracted needle hub.