A syringe with a blocking structure, and the structure includes a one-way engagement unit having a propping portion, a flange extended outwardly from a rod and abutted with a cylindrical body, and a flexible bracket extended outwardly from the rod and isolated from the propping portion, and the flexible bracket has an external diameter greater than the propping portion. When the rod is moved forward with respect to the cylindrical body to pass through the flange, the flange abuts the flexible bracket to tilt backward. When the rod is moved backward with respect to the cylindrical body, the flange abuts the flexible bracket to tilt the flexible bracket forward, and the propping portion blocks the flexible bracket.
