An anti-back mechanism for a drug delivery device according to the present application includes a retaining groove (7), a position retaining protrusion (6) and a clamping groove. The retaining groove (7) is provided on a push rod (4). The position retaining protrusion (6) is provided in the drug delivery device. The clamping groove has an accommodation cavity into which the position retaining protrusion (6) is inserted. The position retaining protrusion (6) is provided on a ring having a notch and may be clamped with respect to the push rod (4) when the position retaining protrusion (6) is squeezed. When the position retaining protrusion (6) is subjected to a reverse thrust from a cartridge holder (1) of the drug delivery device, the position retaining protrusion (6) may move in a direction opposite to an injection direction, be inserted into the clamping groove to be clamped, and be hooked to the retaining groove (7) to be locked. For the anti-back mechanism, since the cartridge holder of the drug delivery device is squeezed, the position of the position retaining protrusion (6) is changed, the position retaining protrusion (6) is pushed into the clamping groove and clamped to the retaining groove (7), finally, the position retaining protrusion (6) is hooked to the retaining groove (7) to be locked, thereby achieving the effects of fastening, anti-reversing and clamping. Hereby, a phenomenon of reversing of the push rod (4) is prevented from occurring.
