The invention discloses a large-diameter screw thread disassembling and assembling device, which comprises a cross beam. One or more pairs of fixed pulleys are arranged on the lower side of the crossbeam; a steel wire rope sequentially winds on each fixed pulley; a pin shaft is connected to one end of each steel wire rope; a connecting plate and a bearing are sequentially arranged on each pin shaft from top to bottom; a workpiece A to be in threaded connection with a workpiece B is fixed to the lower side of each connecting plate; and a balance piece is connected to the other end of each steel wire rope, and is used for balancing pressures on two ends of each steel wire rope. The invention further discloses a disassembling and assembling method implemented based on the large-diameter screw thread disassembling and assembling device provided by the invention. The large-diameter screw thread disassembling and assembling device, and the disassembling and assembling method thereof provided by the invention effectively solve the problem so as to realize smooth disassembling and assembling of a large-diameter screw thread.