The invention belongs to the field of machining and particularly relates to a machining method for realizing the coaxiality of bearing positions at two ends of a differential casing. The method comprises the steps as follows: a machining lathe for external circles of mounting positions of bearings at the two ends of the differential casing is prepared, a semi-finished differential casing with a central hole is prepared, is positioned on the machining lathe, is cut by two first cutters simultaneously, is taken down and is clamped on a differential casing inner gear mounting ball cambered surface machining lathe, a cutter is mounted, a differential casing inner gear mounting ball cambered surface is machined, and the cutter is retracted after machining is completed. With the method, the machining precision and the machining efficiency are improved, the rejection rate is greatly reduced, and the common problem of large difficulty in machining the external circles of the mounting positions of the bearings at the two ends of the differential casing and the differential casing inner gear mounting ball cambered surface with the traditional method is effectively solved.