The helical-labyrinth seal (HLS) can reduce rub between labyrinth teeth and rotor during the rotor vibration because the helical teeth on the stator and steps (or teeth) on the rotor are staggered in some positions. The helical-labyrinth seal with the bristle pack named as the helical-labyrinth-brush seal (HLBS) has excellent sealing performance, but the study on the leakage flow characteristics of the HLBS is not available. This paper, using computational fluid dynamic (CFD) analysis technology based on a porous medium model, investigates the leakage flow characteristics of two types of HLBSs (bristle pack installed upstream or downstream of helical-labyrinth tooth named as HLBS-U and HLBS-D, respectively) at various pressure ratios (1–1.3) and rotational speeds (0–10,000 r/min, surface speeds range from 0 to 209 m/s). The radial clearance between the rotor and the bristle pack ranges from 0 mm to 1.0 mm, and the radial clearance between the labyrinth teeth and the steps on the rotor is 1.6 mm. In parallel, the leakage flow characteristics of the HLBS-D with the constant of 1.0 mm are experimentally measured at the pressure ratio up to 1.3 and rotational speed up to 2000 r/min (surface speed 42 m/s). The CFD-derived leakage flow rate (represented as effective clearance) and static cavity pressure agree well with the experimental data in the whole range of test conditions. The shaft rotation eliminates the leakage hysteresis effect of the HLBS-D. Compared with the HLBS-D, the effective clearance of HLBS-U is less sensitive to rotational speed changes. The effective clearance of the HLBS-U is smaller than that of the HLBS-D in the case of = 0.5 mm and rotational speed n < 10,000 r/min, and the case of = 1.0 mm. However, for the case of = 0.5 mm and n = 10,000 r/min, and the case of ≤ 0.1 mm, the situation is opposite. The brush seal sections of the HLBS-U and the HLBS-D offer over 55% and 65% total static pressure drop in the case of = 1.0 mm, respectively. The brush seal sections of two HLBSs bear almost the same static pressure drop of the over 97% total static pressure drop as equals to 0.1 mm. What is more, the HLBS-U has lower turbulent kinetic energy upstream of the bristle pack than the HLBS-D does, which means that the intensity of the bristles flutter of the HLBS-U is lower.