The invention discloses a control system for on-line calibration of drawn fibers and a controlling method thereof. The control system comprises an optical fiber diameter instrument control system and a servo control system which are independently placed and do not contact each other; the optical fiber diameter instrument control system includes a rod feeder control system, a bare fiber diameter measuring instrument, a coating diameter gauge, a cup coating system and a traction system, wherein each system is sequentially fixed at the same side of a fiber drawing tower from top to bottom; the servo control system includes a programmable logic controller and an upper computer; the rod feeder control system comprises a signal receiving system and a signal processing system; and the optical fiber diameter instrument control system uses PLC to realize real-time signal transmission with an optical fiber diameter instrument system and the cup coating system. The control system provided by the invention employs original independently-programmed program of the company, has the advantages of reasonable structure, convenience, practicality and unique perspective, guarantees accuracy of optical fiber diameter measurement through transformation of the components of a conventional drawing system, and improves production efficiency and product quality.