The invention discloses a composite milling material increasing and decreasing type in-situ repairing device for a deepwater pipeline. The device comprises a repair cabin, an equipment cabin, a cutterstorage, a local control system, a six-axis robot, a material increase repair system, a material decrease handling system and a monitoring system. According to the device, an underwater high-pressuredry method is utilized to carry out underwater metal additive manufacturing, so that repaired parts have fewer holes, fewer cracks, high density, high strength and good mechanical property. The plasma melting deposition and laser melting deposition combined material reduction manufacturing are adopted and meet repair work under various conditions; the problem of interface combination of a first layer of plasma melting deposition is solved in a laser pre-deposition mode, and the repair of dissimilar materials is realized; after the repair is finished, an anti-corrosion layer is subjected to laser melting deposition, thereby greatly improving the quality of the repaired parts.