Precision turning with slow tool servo (STS) plays an increasingly important role in advanced manufacturing nowadays. However, it is difficult to promote machining quality for surfaces with local complex geometric features by the conventional global machining method. Hence, a subregional processing method in STS is proposed. First, the continuous equipotential line is taken to express the local geometric feature. Thus, a potential field is built, where the surface could be divided into subregions. Then, a subregional toolpath with variable feed rates is generated by the field and stitched to ensure the feeding motion stability of X-axis. Finally, the surface is subdivided for variable spindle speed planning, considering the feeding motion stability of Z-axis. It is found that the profile arithmetic average error reduces by 31.58% with the proposed method compared with that with the conventional method and the machining time is shortened by 41.00%. Thus, it is proved that the new processing method effectively promotes machining quality and efficiency.