The invention discloses a field machining method of large parts. The field machining method of the large parts includes the processes of part welding, part transferring, part overturning, part field machining and the like. A mounting support, a transferring trolley, an overturning tire, a field machining machine tool and the like are adopted to locate, assemble, weld, move and turn over the large parts so that the large parts can be disassembled into multiple small part bodies to be transported to the use field in batches, finished products are formed at a field, and field welding, moving, overturning, machining and the like of the large parts are achieved, and the problems that the large parts cannot be transported, or finished product transportation is high in cost and inconvenient are solved; and the field machining method of the large parts is the epoch-making innovation.