The invention relates to an injection pen. The injection pen comprises a refill shell, a push rod, an internal-thread shaft sleeve, a triangular movable racket wheel, a spring and a limiting joint, wherein the push rod penetrates through the refill shell; the internal-thread shaft sleeve, the triangular movable racket wheel and the spring are arranged in the refill shell; the limiting joint is clamped at the lower end of the refill shell; the push rod is provided with screw threads of a certain length; a pawl is attached to the end of the screw threads; the internal-thread shaft sleeve is spirally fixed on the screw threads on the push rod; the triangular movable racket wheel is embedded into the push rod, and is meshed with the pawl on the push rod; the spring is embedded into the push rod; one end of the spring is pressed against the triangular movable racket wheel, and the other end of the spring is pressed against the limiting joint; the limiting joint is connected to the push rod in a sleeving way. The injection pen has the advantages of simple structure, convenience in use and operation, accuracy in injection, low production cost, compact structure and easiness in carrying.