On the basis of analyzing the structure and working principle of MRF brake, braking torque calculation formula of two disc MRF brake is established. Dynamic yield stress changing with the applied magnetic field is analyzed. Braking torque generated by MRF’s viscous changing with disk rotation velocity is also analyzed when magnetic field is zero. Furthermore; braking torque changing with magnetic field intensity is analyzed under a certain rotation velocity. The study provides a theoretical basis for MRF design.