Abstract: Swirl brakes can dramatically change the circumferential swirl flow and the preswirl at the seal inlet. The swirl brake is generally used to improve the stability of seal system. In this paper, the geometric parameters of a swirl brake were studied to evaluate its effect on the static and rotor dynamic performance for the labyrinth seal by employing computational fluid dynamics (CFD) method. Influences of length, clearance and number of the swirl brake, and inlet preswirl ratio were analyzed and compared with the conventional labyrinth seal without swirl brakes. The result showed that the swirl brake significantly reduced the circumferential velocity and pressure at the seal inlet. With the increasing number and decreasing clearance, the efficiency of swirl brakes was improved. While the effect on the circumferential velocity degraded as the swirl brake length increased. Preswirl ratio enhanced the circumferential velocity. In addition, compared with the conventional labyrinth seal, the labyrinth seal with swirl brakes showed greater direct damping, lower cross-coupled stiffness, and resultant larger effective damping. Regarding to the optimization of swirl brake geometry, the effective damping and stability of the seal system showed best performances with the clearance of 0.20 mm, length of 3.25 mm and circumferential number of 90.