The utility mode relates to a part of an automotive main reducer assembly, in particular to a differential mechanism assembly which comprises a differential mechanism left shell, a differential mechanism right shell, a driven bevel gear and a universal joint pin. The driven bevel gear is arranged between the differential mechanism left shell and the differential mechanism right shell, the differential mechanism left shell, the driven bevel gear and the differential mechanism right shell are connected through a set of bolts, the joint of the differential mechanism left shell and the differential mechanism right shell and the joint of the driven bevel gear and the differential mechanism right shell are respectively and evenly provided with a set of bolt unthreaded holes in the circumferential direction, the differential mechanism right shell is evenly provided with a set of threaded holes corresponding to the bolt unthreaded holes in the circumferential direction, and bolts penetrate through the bolt unthreaded holes to be tightly matched with the threaded holes. The differential mechanism assembly is simple to manufacture, few in machining process, low in price, convenient to maintain and replace, and small in bolt consumption number.