The invention discloses a large-radius arc surface metal workpiece processing device and a processing method thereof, and solves the problems of difficulty in processing, low processing precision andthe like of a large arc surface metal workpiece in the prior art. The large-radius arc surface metal workpiece processing device comprises a base and a controller, wherein the base is provided with anupright column and a mechanical sliding table; a power head is arranged on the upright and comprises an indexing mechanism and a spindle box; one side of the indexing mechanism is connected with theupright column, and the other side is connected with the spindle box; a first motor and a pull rod are arranged in the upright; one end of the pull rod is connected with the spindle box through a steel wire rope bypassing a pulley component; the pulley component is mounted at the upper part of the upright column; a worm wheel is arranged between a fixed disk and a rotary disk of the indexing mechanism; the fixed disk is connected with the upright column; the rotary disk is fixedly connected with the worm wheel; the worm wheel is matched with a worm on one side of the upright column; the worm is connected with a second motor mounted on the upright column. With skillful overall design and compact structure, the processing device disclosed by the invention ensures accurate processing and highprocessing quality of a large-radius arc surface metal workpiece.