Abstract: A prestressed adjusting mechanism of a bearing is provided with a first and a second rhombic prism ring which are placed opposite each other with a flexible partitioning ring inserted between them. There are multiple channel holes on the flexible partitioning rings, each containing a steel ball. The adjusting mechanism, after being assembled, is installed in the prestressed arrangement of a bearing. It will generate outward action on both sides by increasing the rotational speed of the spindle. The higher the rotational speed, the larger the reaction that counteracts the prestressed forces of the bearing, whereby, the bearing can sustain a high prestressed force to generate high rigidity and conversely sustains low prestressed force. In this way, the bearing can maintain a relatively longer usage life.