Abstract: The present work established a mechanics model of angular contact ball bearings upon focusing on the high speed spindle. The model can calculate the contact load of every ball and bearing stiffness. A dynamic model of the rotor was developed as well in this work. The model can calculate the contact load of every ball with considering machining precision.With a concrete example, the paper studied the effects of roller precision on the axis trajectory of the bearing, the rotor critical speed, the first three order modes, the unbalanced response and the influences of the radial load on the unbalanced response. The numerical calculation results suggested that the axis of the bearing was an ellipse when all rollers had no size errors. With the decrease in the machining precision, the axis trajectory of the bearing was no longer an ellipse, and the amplitude fluctuation of axis motion trajectory was increased. When the precision and axial load were kept constant, the rotor unbalance response amplitude of shaft end was increased with the radial load. With the decrease in machining precision, the critical speed of the rotor was decreased, and the first three modes did not changed. When the rotor angular velocity stayed the same, the rotor unbalance response amplitude was increased with the decrease in radial load precisions. When the precision was the same, the rotor unbalance response amplitude was increased with the rotor angular velocity.