The invention discloses a mechanical weeding fertilizer-applying pesticide-applying composite working device, and belongs to the technical field of agricultural machinery. The mechanical weeding fertilizer-applying pesticide-applying composite working device comprises a pesticide-applying frame. The pesticide-applying frame is provided with a pesticide-applying component. One downward side of a front part of the pesticide-applying frame is hinged with a connecting rod. A lower part of the connecting rod is connected with a weeding mechanism. The weeding mechanism comprises a fixing housing with an accommodating cavity. The interior of the fixing housing is connected with a shears component. The shears component comprises a fixed blade and at least one moveable blade. The moveable blade comprises a moving part. The moving part is provided with a moveable blade head protruded out of the fixing housing. The fixed blade comprises a fixed part. The fixed part is provided with a fixed bladehead protruded out of the fixing housing. The fixed part is fixedly connected to the fixing housing. The moving part can be reciprocated in the interior of the fixing housing. The moveable blade headand the fixed blade head are superposed. The fixing housing is rotatably connected with a cam. The cam contacts with the moving part. The pesticide-applying component is installed behind the connecting rod. The mechanical weeding fertilizer-applying pesticide-applying composite working device is not affected by a driving route or an arrangement position of the weeding mechanism while used for weeding, and operation is more convenient.