Magnetorheological fluid torque servo (MRT) is a novel torque generating and controlling device. It converts the rotational input of active shearing disc into the torque output of passive shearing disc by controlling the rheological property of the magnetorheological fluid. In this paper, the working principle of the MRT was introduced. The effects of the input rotational speed on the shear stress of the magnetorheological fluid and then the output torque were analyzed. The model describing the relationship of the output torque with respect to the rotational speed of the active shearing disc as well as to other parameters was established. A critical rotational speed was deduced. The factors affecting the critical speed were discussed, and an experiment was conducted to ascertain the validity of the theoretical analysis.