The invention discloses a machining and detecting system of a three-division type large-dip-angle inclined box body and a machining and detecting method thereof. The system comprises a machining center, a finish-milling machine tool, a drilling machine and a measuring instrument, wherein the position is square, a conveying belt is arranged every two of the horizontal machining center, the finish-milling machine tool and the drilling machine, and a conveying belt is arranged between the machining center and the measuring instrument. The method comprises the steps of finish-milling and semi finish-milling, wherein margin is reserved for the semi finish-milling according to the requirement for dye penetrant inspection, milling and combination are alternatively conducted on a housing according to the machining sequence, and the housing is gradually assembled. When the reference axis is too short during coaxiality measurement, and the centers of two holes having the farther distance are used as references to evaluate offsets of the centers of other holes. Devices can be utilized at highest efficiency, floor area can be decreased, the burden of workers can be reduced, time and labor are saved, box body size can be increased, and the assembling precision can be improved, so that the machining process is simple and easy to operate, the box body size precision is improved, the box body is not likely to deform, the measuring errors are decreased, and the accuracy of hole system positioning is ensured.