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This study presents the numerical results for the three-dimensional natural gas-air mixing in a rectangular subsonic channel under the standard datum state. The lower and upper decomposition method was employed to obtain the solutions of the full Navier-Stokes equations capable of describing compressible fluid mixing. Grid was generated by the area orthogonal technique. The results indicate that three-dimensional mixing between Natural Gas (NG) and air has more complicated compressible waves around the injection nozzle. The presence of injection with large speed can suppress the boundary separation happened occurring at the top wall due to the channel expansion, but makes the near-wake flow more complicated. The mixing region can be widened by increasing the injection speed. The recurrence of the vortex structure and the coherent structure can be found from the full Navier-Stokes simulation. It can be concluded that the increase in the injection velocity can improve the effectiveness of mixing of NG and air in the near wake of the injectors.