Helical milling is used to generate holes, in which a tool attached to the rotating spindle traverses a helical trajectory, and the diameter of holes will be larger than that of the tool. Based on the principle of helical milling, this paper establishes analytical model of cutting forces. As the cutter travels on the helical path, intersection between the tool and the workpiece changes continuously, in which chip thickness and direction of the cutting forces will vary simultaneously. The cutting forces are not only direct proportional to the axial depth of cut, but also related to the rotational speed and orbital speed of the tool. Cutting experiment is conducted for the titanium alloy. The result shows that the simulated cutting force can be used to predict the change of cutting force under different conditions.