Fluid auxiliary braking devices can provide braking torque through hydraulic damping and fluid auxiliary braking devices can also convert the vehicular inertia energy into transmission fluid heat energy during braking, which can effectively alleviate the work pressure of the main brake. Traditional hydraulic auxiliary braking devices use transmission fluid to transmit torque , however, there is a certain lag effect during the braking. The magnetorheological fluid(MR fluid) can also be used to transmit torque because it has the advantages of controlling braking torque linearly and responding fast of the magnetic field changed. The temperature of the MR fluid will increase when the vehicle with continuous braking. The changes in MR fluid temperature will cause a bad influence on the efficiency stability of the auxiliary braking. So it necessary to clear about the effect of temperature on MR fluid auxiliary braking torque in order to keep the braking efficiency stability through torque compensated by other factors, such as changing the magnetic field strength.In order to analysis the effect of temperature on MR fluid auxiliary braking torque, this study established the mathematical model of the MR fluid auxiliary braking device through the theory of one dimensional flow theory of hydraulic retarders, and the properties of MR fluid are described based on the Bingham model. This paper researched the change of the properties of MR fluid under the same magnetic field condition with different temperatures, and summarized how much compensated torque is needed to keep the braking efficiency stability. Research showed that when the vehicle with continuous braking, the temperature effect on the braking torque is nonlinear. The braking torque will increase with the increase of temperature under the same magnetic field condition, the braking torque will increase faster at high rotating speed.