The work aims to improve contour precision of surface processing of complex parts based upon post processing technology by optimizing parameters of tool swing angle for five-axis linkage machining. A method of optimizing swing angle was proposed since excess tool posture error arising from non-linear motion of rotating shaft would lead to lower contour precision. First of all, tool posture error arsing from linear interpolation of rotating shaft was analyzed to control swing amplitude and initial position of angle of revolution. Then a new interpolation vector was obtained as tool-axis vector after linear interpolation was projected onto the plane theoretically consisting of vectors in initial and end point. Finally, it was verified by performing simulation and actual experiment using an impeller specimen. After the swing angle optimization method was applied, contour error of blade contour and theoretical contour was reduced from 0.08 mm to 0.04 mm, and the maximum overcut volume was also reduced from 0.03 mm to 0.01 mm. After the tool swing angle was optimized, the contour precision of the complex surface parts could be greatly improved. Swing angle of five-axis machine tool is optimized based on post processing technology. Loading angle optimization algorithm is developed based upon general algorithm. Developing a dedicated post processor to process G code program is a feasible measure of improving the contour precision of complicated curved surface. Effectiveness of the method is verified by experiments.