Abstract: The behavior of salting out carnallite from ordinary sea-salt bittern (30°Be,MgCl2/KCl=8, MgS04/MgCl2=0.4) during evaporation has been investigated. In order to avoid separating out of potassium salt during evaporation before the carnallite becomes saturated, it is necessary to keep the ratio MgCl3/KCl in bittern above 10 (at the same time, the ratio MgSO4/MgCl2 below 0.4) by adding the mother liquor of the carnallite to the bittern. But the addition of an excessive quantity of the mother liquor is deemed neither economical nor necessary for industrial production. This paper puts forward a method for calculating the saturation point of carnallite in the settling liquor from the phase diagram, and experiments have been carried out to determine the foregoing saturation point. In order to obtain a stable quality and a maximum yield of carnallite, it is important to keep the settling-separation temperature slightly above the saturation point of carnallite in the settling liquor, and which is, in turn, related to the composition and the final boiling-point of bittern. The solid and liquid components in regard to the cooling method for separation of carnallite have been studied under phase diagram analysis, and the results obtained from experiments coincide with theoretical evaluation. This paper also presents a method for calculating the amount of carallite separated out during cooling and the theoretical maximum percentage of recovery of potassium chloride by adding the mother liquor of carnallite to bittern. It may also be regarded as a proper channel to increase the percentage of recovery of potassium chloride during the process. Finally, the results obtained from plant scale experiments prove that the above statements are correct. As a matter of fact, the yield of potassium chloride from bittern has already reached 87.13% in industrial practice.