To obtain images of high quality from a large space telescope in orbit, an adjustment mechanism for a secondary mirror was developed based on the 6-Prismatic-Spherical-Spherical (6-PSS) parallel mechanism, and the accuracy of the adjustment mechanism was tested. First, the composition and precision requirements for the optical system of the adjustment mechanism for a secondary mirror was analyzed. Next, the error model of the mechanism was established based on its inverse kinematics analysis. Finally, the influences of the structural parameters and position and posture of the dynamic platform on the accuracy of the mechanism were analyzed theoretically. While some structural parameters were selected based on the results of the analysis, others that posed constraints, such as space envelope, weight of the mirror, and random and systematic errors of the mechanism, were analyzed using the Monte Carlo model. Furthermore, a system was developed to test the accuracy of the key technical indicators of the six degrees of freedom (6-DOF) adjustment mechanism for a secondary mirror. The results showed that the displacement resolution, angle resolution, and bidirectional repeatability of the adjustment mechanism were relatively better by 0.1 μm, 0.5″, and sub-micron and sub-arc-seconds order (±0.4 μm/±0.3″), respectively. The absolute positioning accuracy of the adjustment mechanism can be of the order of micron/arc-seconds. It was concluded that the accuracy of the adjustment mechanism for a secondary mirror could meet the needs of large space telescopes in orbit.