The utility model relates to a noise reducing mechanical gear speed changer, which comprises a box body, shafts, a bearing, gears, a gearshift shifting fork and synchronizers. The utility model is characterized in that the box body is divided into a front, a middle and a back parts and has the advantages of good processing manufacturability, easy cleaning and good assemblage performance, and an input shaft, a middle shaft and an output shaft all adopt the supporting of bearing rigidity on the box body, wherein the input shaft and the output shaft are also concentrically connected with each other through the bearing. Every shaft has more reasonable stress situation and can bear bigger load, the synchronizers are dispersed over the input shaft and the middle shaft to realize the full synchronizing gear shifting and power transmission. A constant engaged gear pair is arranged on the middle shaft and the output shaft, and the input shaft is longer than the output shaft. The gears of part of the gears is additionally provided with a gear engaged vibration dampening mechanism to reduce the vibration and the noise.