This paper studies the stratification and erosion phenomenon of helium-air mixture in the CABLE (Containment thermAl hydraulic and aerosol Behavior in Light water rEactor) test facility. The CABLE test facility is equipped with a containment vessel, air/helium supply system, and gas analysis system, and is capable of simulating the post-severe accident conditions inside the containment vessel. The design pressure of the containment vessel is 1.4 MPa, while the inner diameter, the height of the vertical section and the pit are 1.8 m, 4.4 m and 0.6 m, respectively. The free volume of the vessel is 12.94 m3. For experimental safety, helium was injected at the top of the CABLE containment vessel as a substitute for hydrogen produced in severe accidents. The volume concentration of helium was measured by gas analyzers. Ten sampling tubes were installed inside the containment vessel from bottom to top to measure the helium volume concentration at different elevations. With respect to the similar erosion test (SM13) of the SPARC test facility, two scaled-down sets of test conditions were designed and conducted. In both conditions, firstly a 30% volume concentration of helium-rich layer was established in the upper region of the vessel. Subsequently, a vertical air jet in the middle of the containment vessel was injected into the helium-air stratification layer at Reynolds number of 39,900 and 14,300, respectively. The experimental data indicated that the stratified erosion process lasted approximately 1247 s under the high-Reynolds-number condition, and the helium volume concentration at each location was around 4.8% after mixing with air; the corresponding Froude number under this condition was 1.97. In contrast, the stratified erosion process was as long as 15,276 s under the low-Reynolds-number condition, which was about 12 times the corresponding duration under the high-Reynolds-number condition, and the final concentration after mixing uniformly was around 0.5%; the corresponding Froude number under this condition was 0.66. The Froude number was defined as the ratio of the inertia force of the impinging jet to the buoyancy force of the stratified layer. That is to say, the momentum of the air jet had a crucial influence on the erosion speed of the helium layer. The experimental results also showed meaningful inspirations for the scaling design requirements for stratification and erosion phenomenon of light gas mixing atmosphere inside containment.