The work aims to study influence of tool feed rate smoothness on surface roughness and contour accuracy of complex freeform surfaces in five-axis machining. Firstly, the principle of spatial linear interpolation in operation of five axis machine tool was analyzed, and mathematical model was derived for velocity component of each axis in the interpolation period. According to different modes of velocity instruction and actual movement distance of the tool in the numerical control system, actual velocity calculation model was built for spatial movement of tool in terms of end milling and side milling. Then speed and acceleration component of each axis were checked and processed according to the maximum speed and acceleration constraints of each axis on the machine tool, and actual synthetic speed of the tool was finally obtained. Finally, based on post-processing technology, special post-processing software as-developed was used for source code post processing of cutter location, and an impeller test piece was used for verification, and experimental results were analyzed. In the process of complex surface machining, high surface quality and contour precision were obtained at stable surface feed rate. Changes in feed rate had great effects on machining quality if surface curvature changed greatly. Under the same cutting conditions, contour error of blade leading and trailing edge was reduced from 0.1 mm (at constant feed rate) to 0.04 mm (at constant surface speed). In five-axis machining, the more stable the surface feed rate is, the higher the surface quality and the contour precision are. For the complex surface exhibiting great curvature change, it is necessary to strictly control feed rate of tool and obtain stable surface speed as far as possible, so as to reduce overcut value and improve surface quality of parts.