Abstract: Increasing the strength of the gears with the basic size of the gears remains unchanged has always been an important research direction. In this paper, for the ambiguity and complexity of the research and definition of root transition arc of the spiral bevel gear. An accurate method for the root transitional arc of spiral bevel gear is proposed. This method studies the tooth profile curve of the large end face of the equivalent gear of the spiral bevel gear, improve the root arc transition curve and come up with a design method about root transition arc based on cycloid.