An injection pen, comprising a pen holder (6)； the front end of the pen holder (6) is provided with a medicine bin (13)； the rear end of the pen holder (6) is provided with a trigger (1)； a dosage cylinder (3), a differential rod (5), and a transmission rod (10) are coaxially arranged in the pen holder (6) from outside to inside in sequence； the transmission rod (10) is connected with a key slot of the differential rod (5)； the inner wall of the pen holder (6) is provided with a limiting protrusion matching the transmission rod (10)； the front end of the dosage cylinder (3) is provided with a transmission clutch (8)； the rear end of the transmission clutch (8) is provided with a latch； the transmission rod (10) is provided with an inner ring gear matching the latch； the transmission clutch (8) and the dosage cylinder (3) can be linked unidirectionally and transmittingly； the front end of the dosage cylinder (3) is further provided with a push rod base (9)； the front portion of the push rod base (9) is provided with a push piece (11)； a push rod (4) matching the push piece (11) is coaxially provided in the push rod base (9) in screw-thread fit fashion； and the push rod base (9) and the transmission rod (10) can be linked unidirectionally and transmittingly. The injection pen is easy to operate, and can improve the accuracy of dosage control.
