Abstract: A disposable safety syringe has a barrel, a connector assembled at a front end of the barrel, a needle assembly assembled onto the connector, and a plunger extendable into the barrel. The plunger has a connecting head such that the connecting head is able to be securely connected with the connector after an injection via the syringe. Therefore, the needle assembly is able to be pulled by the plunger to fully retract into the barrel and the syringe is then safe to discard without the risk of scratching someone. Moreover, the syringe is provided with a guiding piece to guide the plunger to connect with the connector such that the operation of the syringe is convenient.