【Objective】In order to solve the problem of low magnetic field utilization of traditional magnetorheological brake (MRB), a drum MRB with internal and external fluid flow channel was designed.【Method】By adding non-magnetic rings and non-magnetic disk to the magnetic material rotary sleeve and the stator magnetic cylinder, the magnetic flux was guided meander through six effective damping gaps in the internal and external axial flow channels. Therefore, the torque performance was improved while keeping the outer dimension of the brake remains unchanged. The structure and working principle of the internal and external fluid flow MR brake were described, and the mathematical model of braking torque were deduced and established. Based on the analysis of electromagnetic field and torque, the accuracy of the model was predicted through theoretical calculation and DOE experiment orthogonal method, and the multi-objective optimization of the MR brake with internal and external fluid flow channels was carried out by using the NSGA-Ⅱ algorithm.【Result】The results show that,with an applied current of 2.0 A, the braking torque of the initial and optimal MRB dampers are 36.38 N·m and 47.35 N·m, respectively, which improves by 30.15%. Compared with the initial damper, the braking torque increased by 21.31 from 18.28, and the dynamic adjustable range improves by 16.58%.【Conclusion】The optimal MRB meets the braking performance requirements of unmanned delivery vehicles.