In order to overcome the shortcomings of serious heat in the existing magnetorheological fluid(MRF) brake,a composite braking device of MRF and shape memory alloy(SMA) was designed.The heat generated by the device in the braking process was used to make the SMA spring produce additional friction torque and improve the braking torque of the device.The structure and working principle of the composite braking device were expounded.Magnetic field finite element analysis,SMA spring extrusion force analysis and composite braking torque analysis were carried out.The results show that the torque transmitted by MRF increases with the increase of current,and the friction torque of SMA spring increases with the increase of temperature;when the coil current is 3 A and the temperature reaches 100 ℃,the composite braking torque increases by about 31.4% compared with the single magnetorheological braking torque.