Abstract: To investigate the effect of spindle speed on workpiece surface roughness in contour milling, a corresponding machining and testing system was established. With spindle speed as the variable and other process parameters kept constant, four groups of comparative experiments were carried out at rotational speeds of 2 000, 3 000, 4 000 and 5 000 r/min, while vibration signals of the workpiece were collected synchronously. The experimental results show that with the increase of spindle speed, the cutting force decreases gradually, and both the maximum amplitude and average amplitude of the workpiece are reduced. The sliding distance of the cutting edge on the workpiece surface is shortened, and the sliding friction is weakened, thereby reducing the surface roughness value and improving the surface quality. The conclusions can provide a reference for the reasonable selection of spindle speed in milling.