Secondary mirror adjustment mechanisms are core elements of active optics used for the adjustment of space telescopes. For real-time rapid adjustments in orbit, great demands are put on their adjusting accuracy and dynamic characteristics. Because of nonlinearity, uncertainty, disturbances, and coupling of the system, sufficient control cannot be easily achieved by traditional methods based on intermixing PID systems and inverse kinematics. Focusing on this problem, an uncoupled electromechanical model was proposed by transforming the joint couple dynamics into external disturbances. The model uncertainty was represented by the multiplicative uncertainty resulting from the system identification, and the mixed sensitivity function was designed according to the model error bound and performance index. By using the extended method to deal with the imaginary axis pole, the complex dynamic system was transformed into a stacked S/T/KS problem. A robust controller was designed by MATLAB and a digital algorithm was realized using DSP. The improved robustness, disturbance rejection, and dynamic characteristics were verified by both simulations and experimental results, proving that space-reliable application can be achieved.