Abstract: A speed-adjustable self-propelled four-wheel-drive walk-behind lawn mower is provided. The lawn mower may include a cutting deck, an engine installed on the cutting deck including an engine shaft, and a cutting blade installed inside the cutting deck. The lawn mower further includes a first transmission box and a second transmission box. The lawn mower includes an engine pulley installed on the engine shaft, a first combined pulley in communication with the first transmission box, and a second combined pulley in communication with the second transmission box. The lawn mower includes a belt connecting the first combined pulley, the second combined pulley, and the engine pulley. The belt is configured to transmit, via the engine pulley, a driving force from the engine to the first combined pulley and the second combined pulley to rotate the first transmission box and the second transmission box.