To improve the positioning accuracy of CRTS II track slabs applied to the passenger dedicated line, a 6-DOF parallel mechanism is proposed as the positioning mechanism in the track slab fine adjustment system (TSFS), and the corresponding positioning algorithm is presented. After getting coordinates of prisms on slab measured by a total station, the base coordinate system of the parallel mechanism and the slab's body coordinate system can be identified by solving residual equations. According to the target location of a track slab, the parallel mechanism's target configuration is calculated and the slab can be positioned accurately by adjusting only once. Experiments show that positioning errors of track slabs are not more than 0.3 mm.