<p>The tool axis vector of flat-end tool changes drastically during the five-axis machining of complex curved parts, which seriously affects the surface quality of the workpiece. In this research, a method of the tool axis vector construction and optimization of the flat-end tool based on the feasible region of the machine tool rotation axis was proposed. Firstly, a feasible area for machine tool rotating shaft to meet the requirements of machining accuracy is constructed. The shortest path method is used to construct the initial tool axis vector within the feasible range of the machine tool rotating shaft. Secondly, the genetic algorithm is used to optimize the initial tool axis vector. Finally, the B-spline curve-fitting smoothing algorithm is used to smooth each machine tool rotation axis curve to realize the smooth control of the flat-end tool axis vector. The complex curved surface machining example with flat-end tool shows that the method proposed in this paper better improves the kinematics characteristics of the five-axis CNC machine tool and avoids machining interference, ensuring the processing quality of complex curved parts. The method proposed can be a guideline for the certain practical applications.</p>