Slow tool servo (STS) diamond turning technology is an effective method to process freeform surface optical elements. However, in the process of slow tool servo turning, the machining accuracy is affected by the sagittal height and caliber of the surface. Aiming at this situation, by studying the linearization error trend of the constant-angle and constant-arc methods in STS diamond turning, a tool path generation optimization method combining the constant-arc and constant-angle methods is proposed. In order to realize the reasonable planning of the tool path, joint point selection of the new tool path is optimized. The simulation is applied to the sinusoidal grid surface with MATLAB, and the result shows that the tool path optimization method can effectively improve the surface accuracy.