In order to improve the output torque density of the rotary magnetorheological（MR） damper，a design method of the rotary MR damper with higher precision was proposed. The quasi-steady-state flow differential equation of the MR fluid in each channel of the damper was established. The expression of the velocity distribution of the MR fluid was obtained by using the Herschel-Bulkley constitutive model. The calculation method of damping torque and dynamic range of the damper under high-speed conditions were studied. A numerical simulation of the output torque for each channel of the damper was carried out. The results show that under high-speed conditions，as the current increases，the torque enhancement effect of the helical flow mode shows a trend of first rising and then falling，and finally degenerates into a pure shear mode. The prototype was designed and processed，and low-speed and high-speed performance tests were carried out. The test results are consistent with the theoretical calculations. The improved model under zero-field and high-speed conditions reduce the average error by 129.4%，compared with the traditional model，providing a theoretical basis for designing a rotary MR damper with high output torque density.