A syringe with a blocking structure includes a one-way engagement unit 5, 6, 7 having a propping portion 51, 61, 71, a flange 23, 24, 25 extended outwardly from a rod 4 and abutted with a cylindrical body 2, and a flexible bracket 52, 62, 72 extended outwardly from the rod 4 and isolated from the propping portion 51, 61, 71, and the flexible bracket 52, 62, 72 has an external diameter greater than the propping portion 51, 61, 71. When the rod 4 is moved forward with respect to the cylindrical body 2 to pass through the flange 23, 24, 25, the flange 23, 24, 25 abuts the flexible bracket 52, 62, 72 to tilt backward. When the rod 4 is moved backward with respect to the cylindrical body 2, the flange 23, 24, 25 abuts the flexible bracket 52, 62, 72 to tilt the flexible bracket 52, 62, 72 forward, and the propping portion 51, 61, 71 blocks the flexible bracket 52, 62, 72.
