The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) project is one of the National Major Scientific Projects undertaken by the Chinese Academy of Science. There are 16 low resolution multipurpose fiber-fed spectrographs in total, enabling it to obtain the spectrum of celestial objects as faint as down to 20.5. Building autofocusing

systems for the spectrographs is important due to the popularity of instruments. The system enables the optical system to automatically compensate changes in accordance to external variables, such as temperature, timidity, to ensure the spectrum collected more reliably. Image-based algorithm is utilized to calculate the departure of CCD plane from optical focal plane. The calculation also aids to regulation of the system. The defocus value is transformed to the controlling computer of each spectrograph. A driving step-motor performs refocusing function by moving the fiber slit unit to its right position.