Abstract: The FAST (Five-hundred-meter Aperture Spherical radio Telescope) now being assembled will be the largest and most sensitive single-dish radio telescope in the world. The FAST uses an Arecibo-type antenna of three outstanding aspects: its location within a Karst depression, its being an active reflector antenna, and its light-weight feed cabin. During observational tracking its reflector surface will be adjusted by down-pull cables and main cables, which are driven by actuators. The actuators consist of mechanical, electrical, and hydraulic components. The actuators will work in a high-humidity depression and constantly bear pulling forces with the maximum levels ranging from 6 to 10 tons. The actuators, which number in 2225 sets, have an average speed of about 0.2mm/s, and some actuators will work continuously. It will thus be inevitable for some actuators to have faults. Actuator faults will reduce service efficiencies and reflector-surface accuracies in the long lifetime of the FAST which is designed to be 30 years. However, the distance between any two of the actuators will be more than 10m, and the actuators will be distributed over the bottom of a 500m depression with some of them located in steep rugged areas or being blocked by down-pull cables, other actuators, flanges, and anchors. These factors make it hard for field maintenance of actuators. A mechanism for rapidly dismounting and assembling a faulty actuator will be an indispensable tool for normal operation of the FAST. We hereby propose a design of such a mechanism. We present several key aspects of the design, including easy installation of connections to actuators, safe force loading, manual adjustment of the pull-rod length by staff without being lifted above ground, and a ladder with dismountable pins. We analyze two particular issues of the design, The first is a finite-element analysis of the strength, stiffness, and weight-minimization of the open C-type upper-connecting rod. The second is control of moving shear-connector components. This study will provide design schemes,analysis approaches, and calculation methods for development of similar mechanisms in future.