Abstract: A prefilled injection pen for injecting insulin, the prefilled injection pen including: a screw rod, a memory connection barrel, a fixed barrel and a rotating barrel. A plurality of first one-way teeth is evenly distributed on an end surface of the rotating barrel. The memory connection barrel is disposed inside the rotating barrel. The inner wall of the memory connection barrel is provided with a protrusion matching a thread of the screw rod. An end face of one end of the fixed barrel is provided with a ratchet pawl matching the first one-way teeth. The fixed barrel is also provided with a first inner thread matching the screw rod. The screw rod is provided with two machined plane surfaces. A plunger is disposed at one end of the screw rod and a stopper is disposed in a thread groove at the other end of the screw rod.