The safe syringe comprises a cylinder, a needle seat and a pusher plunger. The needle seat is firmly connected to the front inner side of the cylinder, the needle seat has on its bottom a first protruding piercing portion, a chamber with a positive pressure is formed between the bottom of the needle seat and the front end of the cylinder; the pusher plunger is a hollow rod having a chamber therein with a negative pressure, the rod has a second piercing portion on the outer wall on its front end, and a piston envelops the front surface of the rod and the outer wall. Thereby, when the plunger is pushed to the front end of the cylinder, the first piercing portion pierces and breaks the piston and the front surface of the rod, the second piercing portion pierces forwards through and cut out the bottom of the needle seat to make communication of the two chambers, thus the bottom of the needle seat is drawn into the plunger together with the needle by changing of pressure.