Based on the characteristics of magneto-rheological fluid, a new type of transmission device has been developed in this paper, in which output torque can be controlled accurately by adjusting electric current. It can be applied to replace magnetic particle clutch widely used at present to achieve tension control in some small-sized winders. A structure design of prototype has been proposed. By using the Bingham plastic constitutive model, the output torque of the device has been derived. Electromagnetic finite element analysis has been employed to simulate the magnetic circuit of this magneto-rheological fluid tension control device. It is shown that this magneto-rheological fluid tension control transmission has its significant advantages comparing with traditional magnetic particle clutch. The new one can produce higher controllable torques, and may be applied widely in the future.