Main mirror in LAMOST is a spherical mirror with 4 meters effective aperture, it is assembled by 37 hexagonal segments, and the orientations of these segments were adjusted by actuators to achieve optical co-focal status. Arithmetic for facing control of segmented-mirror was pre-researched in this paper, based on present condition in LAMOST. To maintain the main mirror in a facing figure and preserve it during the track is the core of this control. To achieve the facing figure, the unique method in segmented-mirror technology nowadays is that sensors working with actuators to form closed loop. Firstly, relationship between the measurement results of sensors and the movements of actuators was calculated and the figure control equation set was founded. Secondly, the characteristic of the coefficient matrix of this equation set was analyzed. Finally, several methods to solve this equation system were comprehensively analyzed. Damped Least-Squares Solution (DLS) was selected as the best for this paper, and this method was programmed to apply on the experiment of sub system, finer result was got. Petal-effect in the experiment was noticed, analysis was given to show the control of whole main mirror would get rid of petal-effect.