Backflow proof structure for syringe

A backflow proof structure for a syringe comprising a central cylinder portion and two blocks positioned in a protruding end of the syringe, on the bottom of the cylinder portion is a flared portion and on the top thereof is a tip, the outer space of the tip is filled with a piece of crystallized syrup to separate a crown cap covering the top of the tip from the cylinder portion; when the cylinder portion is placed in the protruding end, and a silicone cube is placed there-beneath, an irregular-shaped stopper is placed to closely contact the bottom of the protruding end to push the crown cap on the top of the cylinder portion out of the protruding end. When the syringe and the needle are connected with each other and the cylinder portion is pressed down, the crown cap is pressed into the protruding end to form a passage; when the pressing force disappears, the silicone cube bounce the cylinder portion back to again let the flared portion block the outlet of the protruding end, this effectively prevent backflow of the injection liquid or its contact with other pollution sources.