A retractable safety syringe includes a retractable needle hub holding a needle and having a first guiding means; and a hollow barrel having a second guiding means set correspondingly to the first guiding means; and a collapsible plunger comprising of a first plunger element having a protrusion releasably coupled with a second plunger element having a longitudinal slot with a pinched zone to curb the movement of said protrusion; and a spring disposed between the needle hub and hollow barrel and acts between the needle hub and the hollow barrel. The uncoupling of the collapsible plunger triggers the retraction mechanism to enable the needle hub to retract into the barrel by the decompression force of a spring. The reliability of retraction is further improved by a longitudinal slot and a protrusion to facilitate the retreating of the collapsible plunger in an orderly way.