Abstract: The compressibility of gaseous mixture was usually neglected when doing CFD（Computation Fluid Dynamics） numerical simulation for low-speed gas flow. However, the non-compressible approximation is no longer valid when the temperature difference or the concentration difference is too large in the computation system and the computational error is unacceptable. A Low Mach Algorithm is developed to solve such a problem. By dividing the actual pressure into a Thermodynamic Pressure and a Mechanical Pressure, the algorithm combines the thermodynamic pressure with the physical properties and is able to simulate the low Mach compressible flow more precisely, with relatively tiny modification to the present incompressible solver. The algorithm was realized on Code_Saturne and was applied to simulate the erosion phenomenon of light gas stratification by air injection. In this paper, the mesh refinement sensitivity is firstly performed and the mesh with medium refinement is selected for the following computations. Then, after a comparison among the results by different algorithms, it is found that the Low Mach Algorithm improved the precision significantly. At last, with the obtained simulation results, the horizontal distribution of helium in the experiment is also analyzed and discussed.