The invention discloses a large-size workpiece plane circular-arc groove machining device and a method for milling a circular-arc groove by using the device. The large-size workpiece plane circular-arc groove machining device comprises a bottom plate, a circular-arc track feeding device, a milling depth adjusting device and a milling cutter guide device, wherein the milling depth adjusting device is connected with the circular-arc track feeding device; the milling depth adjusting device is used for realizing feeding adjustment of a milling cutter in milling depth; the circular-arc track feeding device is fixed on the bottom plate so that the feeding of the milling cutter in the peripheral direction is realized; the milling cutter guide device and the circular-arc track feeding device are arranged in parallel and are connected in the horizontal direction; the horizontal distance between the central line of the milling cutter guide device and the central line of the circular-arc track feeding device is equal to the machining radius of a circular arc to be machined; and a checking arc groove is formed in the bottom plate. According to the large machining device and the method for milling the circular-arc groove, a large-size workpiece plane circular-arc groove can be machined and manufactured in the field and the machining requirement that the depth of the circular-arc groove is less than or equal to 0.5mm is realized; and the large-size workpiece plane circular-arc groove machining device has a simple structure, is convenient to operate, is economical and practical, and is low in cost and high in machining precision.