The invention discloses a method for removing the temporary hardness of underground water and a device for achieving the method. The method comprises the steps that raw water and hydrochloric acid are mixed to obtain water where CO2 gas is dissolved, the water where CO2 gas is dissolved and air are subjected to air-water mixing to obtain water containing small air bubbles, the small air bubbles in the water and the CO2 gas dissolved in the water are combined to float out of the water surface, so that CO2 is dispersed into air along with the small air bubbles, and the CO2 gas in the water is removed. The device comprises a water collecting and air stripping area and an air-water mixing area which are sequentially arranged from top to bottom, and an air-water mixer is arranged to be connected with the water collecting and air stripping area and the air-water mixing area. The device is simple in structure and small in occupied area; the reaction theory is simple, operation is convenient, and a high water scale removing rate is achieved on the underground water with the large temporary hardness.