The invention discloses a processing method for a parallel threaded hole in an inner cavity of a large shaft. A special mould is fixed in the inner cavity of the shaft and the parallel threaded hole in the inner cavity is processed by means of a magnetic drill, so that the problem that the threaded hole cannot be processed by large equipment is solved. The mould is not welded and fixed to the inner cavity of the shaft, so that the problem of deformation caused by welding the inner cavity of the shaft can be solved. The processing precision is improved, the processing efficiency is improved greatly and the manufacturing cost is saved. Meanwhile, the process method has the advantages of being simple and convenient, saving the manufacturing cost, shortening the production period, being high in processing precision and the like. The processing method is suitable for processing all parallel threaded holes in the inner cavities of the large shafts, and has very good promotional and application value.