The utility model discloses a milling machine, as well as a belt tightening pulley device and a belt pulley transmission mechanism thereof. The belt tightening pulley device comprises a pulley axle, a tensioning pulley, a plurality of bearings, a mounting seat provided with a through hole, a gasket and a lock nut, wherein the tensioning pulley can move in the self axis direction. Compared with the prior art, the belt tightening pulley device has the advantages that in the assembly process of a driving pulley, a driven pulley and the tensioning pulley, at the beginning, the pulley grooves of the driving pulley, the driven pulley and the tensioning pulley are not needed to be in the same surface, only the pulley grooves of the driving pulley and the driven pulley are needed to be in the same surface, and then the pulley groove of the tensioning pulley can automatically adjust the position under the floating self adjustment of the tensioning pulley, and is enabled to be positioned on the surface where the pulley grooves of the driving pulley and the driven pulley are. Therefore, the assembly process is allowed to be simple, time and labor are saved, the condition that the pulley grooves of the three pulleys are in the same surface is relatively easily realized, and the abrasion of a transmission belt is reduced.