A locking structure for an injection pen. The locking structure for an injection pen is disposed among a spiral duct (30) a push rod (81) and a propelling lever (60) of a circularly pushing device (1) of the injection pen. The propelling lever (60) penetrates in a tube center hole (331) of a linkage wall (33) in the spiral duct (30) by means of a dosage control section (63) having a push thread (631). The push rod (81) penetrates in the propelling lever (60) by means of a rod portion (811). The locking structure comprises a necking ring portion (36) a buckling recessed portion (814) and an elastic clasp (612). The necking ring portion (36) is formed at the rear side surface of the linkage wall (33) in the spiral duct (30). The buckling recessed portion (814) is formed in the front section of the rod portion (811) of the push rod (81). The elastic clasp (612) is formed in the rear end of the propelling lever (60) adjacent to the dosage control section (63).
