The invention relates to a machining method for the cross-shaped shaft holes of differential mechanism semi-shells, which is characterized in that the differential mechanism semi-shells are arranged in the inner cavity of a fixture by being positioned with the external circle of the lower end and the bottom end surface of the differential mechanism semi-shells and the fixture is fixed on a swiveling operating platform; semicircular holes are machined successively through the modes of chambering, heavy boring and refined boring; the swiveling operating platform is arranged so that the differential mechanism semi-shells automatically rotate at 90 degrees; and the four perpendicular semicircular holes are machined successively according to the sequence. The machining method carries out the machining on the swiveling platform of a vertical machining center so as to carry out arbitrary scale division; the oil pressure type locking is adopted so that the clamping is reliable; the high-precision tooth type positioning is adopted to accurately rotate the differential mechanism semi-shells at 90 degrees and ensure the coaxiality and the verticality among the four cross-shaped shaft holes; the designed chambering cutter, and heavy and refined boring cutters guarantee that the machining dimension precision is high and stable; and the positioning fixture is designed and is fixed on the chuck of a machine tool so that the process reference and the design reference of the differential mechanism semi-shells are consistent. Compared with the traditional process, the machining method of the invention saves the assembling and calibrating working procedures of the two semi-shells and enhances the production efficiency so that the products have interchangeability.