The beam director is the core component of the optical system, and the gas sealing of the internal optical transmission channel plays a decisive role in the overall performance of the system. To address the issues of easy wear and significant relaxation in traditional packing seals, this paper designs a beam director sealing structure based on magnetorheological fluid and studies the working principle and theoretical model of magnetorheological fluid sealing (MRFS) technology. Firstly, a MRFS structure with permanent magnet array and anti-centrifugal type is designed to meet the specific requirements of large diameter seal of beam directors. Secondly, the observation experiment of the rheological properties of the micro-magnetorheological fluid is carried out to analyze the mechanism of MRFS at the micro-scale. At the same time, the macroscopic rheological properties are tested based on rheometer, the yield model of magnetorheological fluid based on magnetorheological complex field is fitted, and the modified MRFS pressure resistance formula and friction moment model are further established. Finally, the equivalent experimental platform of the MRFS of the optical path of the beam director is built, and the pressure resistance test and friction torque test were carried out under different temperature conditions, rain and salt spray simulation conditions. The experimental results show that the MRFS can fully meet the requirements of the optical path seal of the beam director, and the modified model can effectively reflect the influence of temperature on the performance of MRFS. It provides an important theoretical basis and practical guidance for the innovation of the optical path seal technology of the beam director shafting.