The wall-slip effect is observed in areas with magnetorheological fluids (MRFs). A slip layer is formed, which reduces the friction between the solid particles and working surface that causes relative movement of the particles. This leads to errors in the measurement of rheological parameters and an inaccurate braking torque model. Thus, here, a rheometer with a sandpaper on the rotor is used to change the working surface roughness to analyze the wall-slip effect of the MRFs. Based on the experimental results, the influence patterns of wall-slip effect on fluid viscosity and yield stress are obtained. Furthermore, a MRF model is established that considers wall-slip effect, which is different from the conventional models. The model is employed to establish a magnetorheological (MR) braking torque model. To verify the braking torque model, a prototype was manufactured, and its mechanical properties were tested. When compared with a smooth rotor, the braking torque of MR brakes with rectangular grooves is increased. This confirms the existence of the wall-slip effect and shows that the wall-slip effect of MRF can be effectively suppressed by incorporating grooves on the rotor surface.