The invention discloses a method for softening high-hardness waste water through gas and liquid combination. The method for softening the high-hardness waste water through gas and liquid combination comprises the following steps that a softening agent and the high-hardness waste water are fully mixed, so that a mixed solution is obtained; the mixed solution is made to make contact with carbon dioxide gas so as to react with the carbon dioxide gas; solid-liquid separation is conducted on the mixed solution, and then treatment of the high-hardness waste water is completed. The device for softening the high-hardness waste water through gas and liquid combination comprises a gas and liquid combination spray tower, a micro-nano aerating system, a horizontal flow sedimentation tank and a sand filtration tank, wherein the micro-nano aerating system is arranged at the bottom of the gas and liquid combination spray tower, the horizontal flow sedimentation tank is communicated with a water outlet located in the upper portion of the gas and liquid combination spray tower, and the sand filtration tank is communicated with an overflow pipe located on the upper portion of the horizontal flow sedimentation tank. The device is simple in structure, high in treatment capacity and high in treatment efficiency; when the device is applied to the method for softening the high-hardness waste water through gas and liquid combination, the device has the advantages of being low in cost, good in treatment effect, simple in technique and the like.