Based on the geometric analysis of the air duct of 3-D impeller, the rough machining boundary and the diameter of cutter are determined, which can be used for the tool-path planning of impeller with long blades or long and short mixed blades. The zig-zag and the triangular tool-path generating algorithms are presented using equally layered strategy. The computing results indicate that the zig-zag method is easy to program with less number of tool retractions while compared with popular one-way method; and the triangular method has the characteristics of shorter length of cutting tool-path and less number of tool retractions, which is very helpful for the improving of machining efficiency. The direction of cutter axis is determined by connecting CC-point and the corresponding point of the shroud surface. The feasibility of these algorithms has been verified by actual examples.