The working principle of motorized spindle unit with AMB was introduced and the impact factors of its rotary accuracy was analyzed. Taking the spindle of engraving and milling machine with the 22 kW power rating and 24 000 r/min as an example, the influence of the spindles suspension accuracy, loads variation and the control systems feedback accuracy on the spindles rotary accuracy was analyzed when the controllers reaction time and the spindle noses cutting force were less than 10 ms and 100 N respectively. The results show that when the spindles roundness error of front and back radial magnetic bearing are between 0 mm to 0002 mm and between -0006 mm to 0006 mm respectively, the loads variation of the spindle nose is less than 1 N, the spindles rotary accuracy meet the requirement. It is significance to improve the working performance of the motorized spindle unit with AMB.