During the LAMOST observation, to accurately align a large number of fibers with the target star positions, we used a closed-loop feedback system based on visual measurement in fiber positioner operation mode. The fiber was illuminated at the end of the spectrometer and the fiber light spots on the other end of the focal plane could be captured by the metrology system for positioning. The system can have a larger field of view and a single measurement can cover thousands of fibers. The metrology accuracy which is based on camera accurate calibration, is critical in the fiber positioning system. In general, calibration of a standard camera requires a reference surface with a known precise position marker and covering the camera's field of view. Theoretically, it is necessary to design a standard target surface that covers the camera's field of view to calibrate the camera's error. However, it is not realistic to manufacture and install a large standard target that meets the accuracy requirement. To ensure that the camera calibration error is within the limited range, and the fiber positioner can obtain higher positioning accuracy, we use the focal plane unit hole to insert a dedicated reference unit to serve as its calibration reference. In this paper, a reference fiber unit structure was designed according to the requirements of closedloop positioning. Through the test experiment on the reference fibers, it was finally verified that the reference fiber unit meets the accuracy requirements of closed-loop control.