The utility model relates to an injection pen. The injection pen comprises a shell, a knob, a connecting
pipe sleeve, a first elastic piece, a push rod, a retaining
assembly and a
medicine bin, the knob is rotatably connected to the shell; the connecting
pipe sleeve is positioned between the shell and the push rod; the push rod is connected with the retaining
assembly; the first elastic piece is connected to the connecting
pipe sleeve; a tooth structure is arranged on the inner wall of the shell; a connecting part and a
side arm are respectively arranged at two opposite ends of the connecting pipe sleeve; in a
dose setting stage, the knob is connected with the connecting part to drive the connecting pipe sleeve to rotate synchronously, so that the first elastic piece rotates to store energy; the
side arm is kept in contact with the tooth structure so as to prevent the first elastic piece from releasing energy; after injection is triggered, the connecting part is separated from the rotary knob, the
side arm is separated from the tooth structure, and the connecting pipe sleeve moves to the position connected with the maintaining
assembly, so that energy of the first elastic piece is released, and
medicine in the
medicine bin is pushed. The injection pen is compact in overall structure, and the reliability of the injection pen can be improved.