A developing cartridge, detachably mountable on a photosensitive drum cartridge, the photosensitive drum cartridge being detachably mounted inside an image forming apparatus, wherein the image forming apparatus includes a force-applying component for applying a force to the developing cartridge; the photosensitive drum cartridge includes a drum holder and a photosensitive drum mounted on the drum holder, the drum holder is provided with a supporting part (345, 345a) for mounting the developing cartridge; the developing cartridge includes a frame and a developing roller mounted on the frame, at least one side-end surface of the frame of the developing cartridge is provided with a supported element cooperating with the supporting part; when the developing cartridge and the photosensitive drum cartridge are mounted inside the image forming apparatus, the supported element receives the force applied by the force-applying component such that the developing roller is maintained pressed against the photosensitive drum; at least one sidewall of the drum holder is provided with the supporting part; and an angle (a) between a direction of the force and a normal direction of a contact interface between the supported element and the supporting part is not smaller than a friction angle between the supported element and the supporting part; characterized in that the force-applying component is provided with an inclined introduction side and an inclined pressing side which incline towards two different directions respectively; the supported element is guided to the inclined pressing side through the inclined introduction side and is subjected to the force applied by the inclined pressing side; and the force generates a first force towards the supporting part and a second force towards the developing roller, the direction of the first force is opposite to the direction of a rotational moment generated when the photosensitive drum drives the developing roller to rotate, the second force makes the developing roller keep pressed against the photosensitive drum when the developing cartridge and the photosensitive drum cartridge are installed in the image forming apparatus. the image forming apparatus includes a force-applying component for applying a force to the developing cartridge; the photosensitive drum cartridge includes a drum holder and a photosensitive drum mounted on the drum holder, the drum holder is provided with a supporting part (345, 345a) for mounting the developing cartridge; the developing cartridge includes a frame and a developing roller mounted on the frame, at least one side-end surface of the frame of the developing cartridge is provided with a supported element cooperating with the supporting part; when the developing cartridge and the photosensitive drum cartridge are mounted inside the image forming apparatus, the supported element receives the force applied by the force-applying component such that the developing roller is maintained pressed against the photosensitive drum; at least one sidewall of the drum holder is provided with the supporting part; and an angle (a) between a direction of the force and a normal direction of a contact interface between the supported element and the supporting part is not smaller than a friction angle between the supported element and the supporting part; characterized in that the force-applying component is provided with an inclined introduction side and an inclined pressing side which incline towards two different directions respectively; the supported element is guided to the inclined pressing side through the inclined introduction side and is subjected to the force applied by the inclined pressing side; and the force generates a first force towards the supporting part and a second force towards the developing roller, the direction of the first force is opposite to the direction of a rotational moment generated when the photosensitive drum drives the developing roller to rotate, the second force makes the developing roller keep pressed against the photosensitive drum when the developing cartridge and the photosensitive drum cartridge are installed in the image forming apparatus.