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).
