An injection device for diesel exhaust fluid includes a cylinder, a stopper, and an air tube. The cylinder encloses a receiving space. The cylinder has an opening at the bottom end. The stopper is pipe-shaped and has a stopping portion and a top portion at two opposite ends thereof. The stopper is arranged in the receiving space and is movable along the longitudinal direction of the cylinder. the stopping portion selectively plugs the opening of the cylinder so that fluid in the receiving space is unable to flow out via the opening. The air tube is inserted through the stopper and the opening of the cylinder to communicate with exterior. The air tube is positioned to the cylinder near the opening so that the air tube may not move with the stopper.