Plunge milling is used to remove excessive material rapidly in roughing machining. The cutter feed in the direction of machine spindle which has the highest structural rigidity. Through the single factor experiment of plunge milling a P20 die steel, this paper investigates the influence of side step, forward step, cutting speed, feed per tooth on cutting force. On this basis, the principles of optimization of interpolation milling parameters are proposed. This guidance of mold production practice has significant reference value.