In mechanical equipments or manufacturing processes, force control is definitely required to achieve desired performance or to ensure product quality. Aiming at this issue, this paper introduced an author-invented torque servo device that utilizes magnetorheological fluids (MRF) to provide a controllable torque. In the paper, the structure and working principle of the MRF-based torque servo device (MRT) were introduced, the constitutive model and the characteristics of the MRF were established and discussed. On the basis, the static and dynamic models of the MRT were both analytically and experimentally studied, and the control of the output torque of the MRT was demonstrated. As an example, the application of the MRT to the polishing of aspheric surfaces was presented in the end.