The present invention relates to a sliding pulley drive mechanism in cutting platform of lawn mower. An engine for the pulley drive mechanism is fixed to a chassis; a cutting platform is connected to the chassis, and can move up and down in relation to the chassis; a bracket is fixed to the cutting platform; cutting platform drive belts are fitted over the output shaft of the engine, an external spline shaft is fixed to the output shaft of the engine, and a belt pulley with an internal spline is mounted on an external spline shaft; the cutting platform drive belt is mounted on the belt pulley with an internal spline; the lower end of the belt pulley with an internal spline is connected with a shock absorbing bushing, a bearing, and a bearing base plate. The bearing base plate is connected to the bracket. In this invention, with a spline mechanism, the belt pulley on the engine can slide up or down on the output shaft of the engine as the elevation of the cutting platform is adjusted, so that the cutting platform drive belts are kept in the same plane, and therefore the belt service lifespan is elongated.