The helium-xenon(He-Xe) mixture has good heat transfer characteristics with a low Prandtl number(Pr ≈ 0.2 ∼ 0.3) and is suitable for the advanced compact gas-cooled reactor. The characteristics of horizontal flow stratification in the fuel rod bundles with He-Xe mixture under gravity and buoyancy were numerically simulated. Different wall heat flux q+ suitable for the reactor operating conditions were selected for calculation and analysis. Flow stratification is observed as strongly inhomogeneous in the radial direction. With the semi-local wall coordinates and Van-Driest transformation, laminarization is manifested by velocity and temperature profile and turbulent kinetic energy changes. In the horizontal flow, where the buoyancy is orthogonal to the direction of flow development, unique characteristics appear in the flow and heat transfer, and the growing longitudinal momentum and energy transport are considered the main reasons. The influence of flow acceleration and buoyancy under different bundle structures were compared. The buoyancy effect is evident in the loosely spaced rod bundles. In contrast, the thermally-induced flow acceleration effect in the tight-lattice scheme is noticeable.