In order to predict and prolong the effective length of the slotting cutter of the double-circular-arc harmonic circular spline, several models are established: 1) the mathematical model of the circular spline tooth profile; 2) the mathematical simulation model of slotting the circular spline by the kinematic method; 3) the mathematical simulation model of grinding the slotting cutter by the tooth profile normal method, to obtain the equivalent tooth profile and the machining error and predict the effective length of the slotting cutter. Based on these models, the effective length of the slotting cutter is prolonged by optimizing the tooth profile of the grinding wheel and the tooth number of the slotting cutter. The results show that before optimization, the effective length of the slotting cutter with 55 teeth is 2.1 mm and the tooth profile error of the circular spline slotted by the 0-cross section slotting cutter is 2 μm. After optimizing the tooth profile of the grinding wheel, the maximum of the tooth profile error of the circular spline slotted by the 0-cross section slotting cutter is 0.14 μm. The effective length of the slotting cutter increases as the number of slotting cutter tooth increases 4.8% for 42 teeth, 52% for 55 teeth, and 81% for 68 teeth.