A constant scallop-height tool path planning method was presented based on envelope surface construction. Firstly, in the process of calculation, the machined surface was offset by the distance of tool radius and scallop-height respectively, then the scallop surface and the tool center surface were obtained. Then the envelope surface, whose section radius was equal to the tool’s, was constructed along the last tool path. The scallop curve was obtained by the envelope surface intersection calculation with the scallop surface. The envelope surface was again constructed along the scallop curve. The adjacent tool path was the intersection line of the envelope surface and the tool center surface. All tool paths could be determined by recurring in turn. A method of boundary verification was described to avoid undercut. Finally the validity of present method was demonstrated by an example. The calculation results of the present method were compared with those of the existed algorithms. The results indicate that the machining precision and efficiency can be enhanced greatly with threeaxis free form surface machining.