The invention belongs to the field of additive manufacturing, and specifically discloses an additive manufacturing method and device for a large metal part. Firstly, the additive manufacturing technology is used, slice processing is performed according to a data model of the part, then contour data of all layers are extracted, a deposition path of a contour is generated, a metal cavity having a certain thickness and depth is manufactured on a substrate, smelted metal is injected into the metal cavity, after the smelted metal is in the solidification or semi-solidification state, the part is rolled and processed, deposition of a new metal cavity in the part is continuously carried out, and the above steps are continuously executed until the overall part is completed. The additive manufacturing method and device solve the defects that a current large-scale metal part wholly adopts the additive manufacturing process so that the efficiency can be insufficient and cracking can be easy, thecasting process is adopted so that defects such as contraction and shrinkage can exist, the shortcoming that the process is complicated and the cycle is too long and the shortcoming that the forging process is adopted so that a forging press can have the tonnage limit and the cycle can be too long, and the high-performance metal parts can be quickly manufactured with low cost.