A method of the digital simulation for the sheet metal CNC incremental forming was proposed based on the Z-map model. The tool extrusion movements were divided into two types according to the principle of sheet metal CNC incremental forming, i.e. vertical movement and horizontal movement. The vertical and horizontal swept volumes of the tool were built to intersect with the Z-map model of sheet metal respectively and the simulation models were generated. The thickness of deformed sheet metal was predicted and the thickness distribution chart was generated. The case studies show that the method can be applied to NC code verification, forming parameters optimization and formability prediction, moreover the system runs stably and reliably in the whole simulation process.