Abstract: The aeroengine has high relative rotation speed between counter-rotating shafts and harsh operating conditions, which puts forward higher design requirements for low-leakage and long-life seal. The structure of rotational carbon fiber brush seal between counter-rotating shafts was designed, and the theoretical equation of critical closing clearance of rotational bristle considering centrifugal effect was derived. A model for solving the flexural deformation characteristics of carbon fiber bristle under centrifugal force and a model based on three-dimensional tube bundle model for solving the leakage and flow characteristics were established to analyze the influence of rotational speed, bristle installation angle and bristle diameter on the critical closing clearance. The leakage and flow characteristics of the rotating carbon fiber bristle brush seal between counter-rotating shafts considering bristles centrifugal deformation was studied. The results showed that traditional labyrinth seal had a large clearance between labyrinth tooth and rotor surface, which led to a good adaptability to rotor radial runout. The service life of labyrinth seal was long, but its leakage amount was higher than that of other seals. The sealing performance of carbon seal was excellent, but its disadvantage was poor abrasion resistance, which led to a short service life. In contrast, the brush seal as an advanced flexible contact dynamic seal, sealing performance and service life was more suitable for application in the location between counter-rotating shafts. When the bristles were installed at outside rotor, the sealing performance decreased with the increase of outside rotor rotational speed. However, when the bristles were installed at inside rotor, the sealing performance was enhanced with the increase of the inside rotor rotational speed. The bristles installed at inside rotor was the preferred structure for the rotational brush seal between counter-rotating shafts. Increasing the inside rotor rotational speed, increasing bristle installation angle and reducing bristle diameter will result in an increase in the critical closing clearance of bristle. Compared with the GH4145 bristle, the T650 carbon fiber bristle showed a reduction in the critical closing clearance of 73.94% to 82.96% at the same rotational speed of inside rotor. From the perspective of sealing performance, T650 carbon fiber bristle was preferred for the rotational brush seal between counter-rotating shafts. With the bristle installation angle increased from 30° to 45°, the critical closing clearance of the T650 carbon fiber bristle was increased by 285.29%. With the increase of bristle diameter from 0.04 mm to 0.10 mm, the critical closing clearance was decreased by 84.04%. From the initial state to the critical closed clearance state, the leakage amount of brush seal was reduced by 74.68%~85.51%, which showed that achieving critical closure state was the key to improving the overall sealing performance of the rotational brush seal between counter rotating shafts.