The ring milling tool has higher cutting efficiency than ball-end milling tool in finish-milling of hardened steel molds. Also , it has better processing technology and quality of machined surface compared with the flat end milling tool. In order to machine ring milling tool accurately, the theory of edge line of ring milling tool is analyzed. According to the geometrical characteristics of rotary cutting tool and the forming principle of spiral, a continuous cutter edge curve model with equal helix angle for ring milling cutter is derived. The blade edge equation mathematical model can reflect the geometric features of circumferential blade curve and fillet curve. Combined with the established model of cutting line, the ring milling tool is grinded by Saacke five-axis grinding machine. The test results for tool geometric parameters indicate that the tool has high grinding precision.