Abstract: The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is an innovative reflection Schmidt telescope. It is designed to have a spectrum acquisition rate as high as several tens of thousands of spectra per night. By using the technique of controllable double-revolving positioning of optical fibers, the LAMOST can reconfigure and fine adjust optical fibers to accurately match directions to objects in minutes. During an observation period, the positioning accuracy of an optical-fiber unit has a major impact on the quality of a spectrum acquired through the unit. Currently, the only retrievable real-time information for positioning accuracies of optical fibers is the feedback about conditions of the driving stepper motor. Such information is incomplete and indirect. It cannot reflect the actual position of an optical-fiber unit. We design a novel system to measure real-time optical-fiber positioning accuracies by taking into considerations the on-site environment of the LAMOST. During observaton intervals, the system records real-time images of positions of optical-fiber units on the LAMOST focal plane using front light sources. It quickly identifies optical-fiber units of relatively large positioning errors with certain analysis, allowing for appropriate adjustments in observation procedures. The system can be integrated into the existing observation and control systems. The system can provide complete and effective information about positions of optical-fiber units on the LAMOST focal plane.Based on information retrieved through the system observation assistants can properly adjust their observation strategies to ensure acquried spectra to be sufficiently accurate. Information provided by the system can also indicate the quality of the acquired spectra, which increases the efficiency of processing spectra.