Abstract: LAMOST requires fast and accurate alignment of 4000 optical fiber units on a 1.75 m-diameter convex focal plane to simultaneously observe 4000 targets, and the positioning accuracy of the unit is very demanding. However, the gear meshing transmission at the eccentric shaft of the unit, the clearance between the shaft and the bearing, etc., can cause the movement of the actual mechanism and the ideal mechanism during the positioning operation of the optical fiber to deviate, and affect the precise positioning of the optical fiber. In this paper, the virtual prototype ADAMS is used to simulate the gear transmission at the eccentric shaft of the unit, and nine groups of comparative simulation tests are designed using the orthogonal test method for different sizes of shaft and bearing clearance. The results of the study show that: The maximum turbulence of the gear at the eccentric shaft is within the allowable range, and the influence on the positioning accuracy of the unit is not significant; The overall error range of the unit shaft clearance is -0.06 deg/sec 0.2 deg/sec, it can make the positioning accuracy of the optical fiber positioning unit worse or even fail to meet the observation requirements. The above analysis and research provide a certain theoretical basis for the design of a new generation of high-precision positioning units in the future.