The present invention discloses a developing cartridge for a laser printer, comprising a stirring wheel and an activation mechanism, the activation mechanism comprising a diameter-changing wheel, a return spring and a sliding block, wherein the diameter-changing wheel is parallel with an axial direction of the stirring wheel and includes two sections of arcs, a larger diameter arc of the two sections of arcs is meshed with the stirring wheel, a protruding block is provided on a lateral end wall at a side of the axial direction of the diameter-changing wheel facing the end cover, a part of the end cover corresponding to the diameter-changing wheel is provided as an inclined wall, the inclined wall is provided with a sliding groove, the sliding block is mounted in the sliding groove, the return spring presses the sliding block in the sliding groove at one end close to the diameter-changing wheel.