Abstract: The control method of a rotary actuator, which is the executive organ of a pneumatic manipulator, was investigated. According to the stability theory, an optimal index function was constructed, and the adjustive law which could ensure the error between the system's output and the expected value tending to zero was deduced based on the index function. Through simulation and experiment, the influence of adjustive parameters on the dynamic performance of the system was also analyzed. The experimental result show that the parameter adjuster based on the stability theory can not only make the expected output of system tracking accurately, but also keep the stability of the closed-loop system.