Abstract: With the assistance of numerical simulation software, the range of “three zones” about spontaneous combustion in the mined-out region that was injected by using CO2 along both sides was studied. By contrast of the range of “three zones” which was measured on spot without CO2 injection, it indicated that the range of oxidation zone could be decreased when the mined-out regions was injected by using CO2 along both sides of it. It reveals that the injection by using CO2 along both sides of the mined-out region can improve the inert range in mined-out region greatly. Then CO2 concentration at outlet corner of working face was measured, which corresponded with the concentration of numerical simulation results. Finally, this paper highlighted optimization methods and safe precautions.