A syringe includes a syringe and a sleeve.The needle cylinder has a cylinder body, the front end of the cylinder body forms a connecting pipe for setting an injection needle, the front section of the cylinder body is provided with an elastic guide plate, and one end of the elastic guide plate forms a buckle part.The sleeve has a pipe body which can be slidably sleeved on the needle cylinder. The sleeve protrudes outward along the axial direction of the pipe body to form a convex strip. The inner wall surface of the sleeve is concave along the position of the convex strip to form a guide groove. When the needle cylinder slides in the sleeve, the buckle part is located in the guide groove and can slide between the first position and the second position. Before and after the convex strip A first limit hole and a second limit hole are respectively provided in the first section, and the front section of the convex strip has a pressing plate corresponding to the first limit hole.一種注射器，包含有：一針筒及一套管。該針筒具有一筒身，該筒身之前端形成一連接管，用以設置一注射針，該筒身之前段設置一彈性導片，該彈性導片之一端形成一卡扣部。該套管具有一管體，用以可滑動地套設於該針筒，該套管沿著該管體軸向向外凸出形成一凸條，該套管之內壁面沿該凸條的位置凹入形成一導槽，於該針筒位於該套管內滑動時，該卡扣部係位於該導槽內，且可於一第一位置與一第二位置之間滑移，該凸條之前、後段分別開設一第一限位孔及一第二限位孔，該凸條之前段具有一按壓片對應於該第一限位孔。
