Through the study of the space tool radius compensation algorithm for flank milling machining, the tool model and the workpiece model were constructed. Solving and formula derivation for them were carried out. The offset was calculated for any flank milling surface to generate a flank milling offset surface with cutter radius compensation, thereby solving the problem of the location of the offset surface during the process of the flank milling machining, and realizing the space tool radius compensation for flank milling machining. The application of this method provides the reference for the numerical control (NC) system to study flank milling cutter radius compensation.