The invention discloses a retractable castor mechanism of a rack. The retractable castor mechanism is installed in the bottom of the rack, and comprises a supporting part, a screw and a universal castor, wherein an accommodating groove is arranged in the supporting part, an opening in the bottom of the accommodating groove is arranged in a base plate of the rack, the screw is arranged in the supporting part, the top of the screw is provided with a screw cap above the supporting part, the bottom of the screw is provided with the universal castor in the accommodating groove, and the universal castor can extend out of or can be retracted into the accommodating groove by virtue of the regulation of the screw. Compared with the traditional castor of the rack, the retractable castor mechanism of the rack, disclosed by the invention, is more flexible and convenient to transport, the installation cost of the rack is reduced, and the installation efficiency of the rack is improved. Therefore the retractable castor mechanism of the rack has the advantages of being simple in structure, easy to process, low in cost, practical, and capable of being applied to most of the racks.