A locking device is disposed between a guiding tube (30), a pushing rod (60) and an injection rod (81) of a syringe. The guiding tube (30) has an annular connection wall (33) and a central hole (331). The pushing rod (60) has a dose controlling segment (63). The locking device has a locking tube (36), at least one engaging face (814), at least one through hole (611), and at least one resilient hook (612). The locking tube (36) is formed on the connection wall (33) and is mounted around the central hole (331). The at least one engaging face (814) is formed on the injection rod (81). The at least one through hole (611) is defined radially in the pushing rod (60). The at least one resilient hook (612) is formed on the pushing rod (60), extends respectively into the at least one through hole (611), and is selectively pushed by the locking tube (33) to abut respectively against the at least one engaging face (814).一種注射筆的鎖定構造，其設於注射筆的循環推進裝置的螺旋導管、推桿及推進桿之間，推進桿以其具有推進螺紋的劑量控制段穿設於螺旋導管內的銜接壁的管中孔中，推桿以其桿部穿設於推進桿中，該鎖定構造包含有縮口環部、扣制凹部與彈性扣鉤，該縮口環部形成於螺旋導管內的銜接壁後側面，所述扣制凹部形成於推桿的桿部前段，所述彈性扣鉤形成於推進桿鄰近劑量控制段後端，當注射筆使用至藥劑瓶內的藥劑用盡時，推進桿移動後的位置使其彈性扣鉤受到縮口環部擠壓而徑向彎曲，使彈性扣鉤伸入推進桿內部扣制推桿的扣制凹部加以制動鎖定，使注射筆的推桿無法再被朝後拉動，避免注射筆過度重複使用，確保使用安全性。
