A medical safety injector, comprising a needle hub (11, 11', 71, 81) connected with a needle (10, 10', 70, 80), a hollow barrel (12, 12', 72, 82) for retaining the needle hub (11, 11', 71, 81) and, after the medical safety injector has completed an injection, guiding the needle hub (11, 11', 71, 81) to retract into the hollow barrel (12, 12', 72, 82); and the medical safety injector is characterized in that the medical safety injector includes a plunger combination (14, 74, 84) disposed within the hollow barrel (12, 12', 72, 82), wherein the plunger combination (14, 74, 84) has a retractable plunger (15, 25, 35, 45, 55, 65) and a hollow shank (16, 26, 36, 46, 56); the needle hub has two ends, and one end of the needle hub receives the needle that extends through a central portion of the needle hub, while the other end of the needle hub is provided with a disc stopper, and a plurality of positioning grooves and a plurality of U-shaped retraction slideways connected with the positioning grooves are provided at proper locations between two ends of the needle hub; the hollow barrel has a front end and a back end, and a plurality of slightly resilient catches are provided at the front end, and the catches are stably snap-fitted in the positioning grooves of the needle hub in a normal unused status and during the injection; at least one helical retractile spring is provided to abut against the disc stopper of the needle hub and is compressed inside a space of the hollow barrel (12, 12', 72, 82) in a normal unused status and during the injection; wherein when a force is applied to the plunger combination (14, 74, 84) and therefore toward the needle after completion of the injection, the at least one protrusion (150, 550, 650) is disengaged from the at least one recess due to the force, and the catches at the front end of the hollow barrel are indirectly forced to expand in a resilient way and thus disengage from the needle hub, and the retractable plunger (15, 25, 35, 45, 55, 65) is forced to slide into the hollow shank (16, 26, 36, 46, 56), and the space of the hollow barrel (12, 12', 72, 82) accommodating the compressed helical retractile spring expands to allow the compressed helical retractile spring to decompress and allow the needle hub (11, 11', 71, 81) to be retracted into the hollow barrel (12, 12', 72, 82) in a direction opposite to an direction of the injection along the U-shaped retraction slideways; characterised in that the retractable plunger (15, 25, 35, 45, 55, 65) and the hollow shank (16, 26, 36, 46, 56) are separately injection-molded from slightly resilient material and put together to allow a bottom portion of the retractable plunger (15, 25, 35, 45, 55, 65) be muff-coupled to a top portion of the hollow shank (16, 26, 36, 46, 56) and thus form a muff-coupled region, and the retractable plunger (15, 25, 35, 45, 55, 65) has an exterior sidewall formed with at least one protrusion (150, 550, 650) in the muff-coupled region for being correspondingly tightly snap-fitted with at least one recess formed on an interior sidewall of the hollow shank (16, 26, 36, 46, 56) in the muff-coupled region in a normal unused status or during the injection, and wherein the retractable plunger (15, 25, 35, 45, 55, 65) includes at least one stress-adjustment cutout (151, 551) provided at a core portion of the muff-coupled region or the hollow shank (16, 26, 36, 46, 56) has an edge (162, 262) provided with at least one stress-adjustment cutout (163,263,563) in the muff-coupled region.