Abstract Curved surface parts are widely used in the industrial applications and the three-axis NC machining with ball-end cutter is the commonly adopted method for certain curved surface parts machining due to its high stiffness and simple operation. With the usage of this approach, the cutting area for different machining positions of the curved surface is changing all the time which results in a severe cutting force fluctuation among the machining process and the cutting force fluctuation is one of the vital factors that affect the machining quality of curved surface parts and the tool life. For the tool-path becomes the main factor that influences the cutting force fluctuation under the condition that the processing condition is determined, an appropriate tool-path planning method with the constraint of cutting force fluctuation for the curved surface machining is crucial for not only guaranteeing machining quality but also prolonging tool life. Based on the cutter location points obtained by the constant scallop-height method, cutting path interval and cutting step length are optimized and the cutter location mesh units are divided firstly, so as to meet the geometry accuracy for the curved surface machining. Taking the nodes of the cutter location mesh units as the cutter location points, the tool-path planning with the constraint of small cutting force fluctuation for the curved surface machining is carried out to improve the surface quality. Taking a saddle surface workpiece as an example, the experimental results show that the novel tool-path planning method proposed in this study has the smallest cutting force fluctuation compared with the other three traditional tool-path planning methods and can truly improve the machining quality of the curved surface parts. This research puts forward an novel tool-path planning approach on account of both the geometric feature of curved surface and the cutting force fluctuation in machining process of the curved surface parts, which provides a guidance for superior quality and high efficiency machining.