The Mengyejing potash deposit in Jiangcheng area of Yunnan Province is the unique ancient chloride type solid potash deposit in China. The main salt minerals include halite, sylvite and carnallite, with sylvite being the most important potassium-bearing mineral. In this paper, the authors used cooling-heating methods to study the homogenization temperatures of primary halite fluid inclusions in order to obtain the information concerning the deposit formation and ancient temperature of the salt lake. The result shows that, in order of white halite layer →grayish sylvite layer →carnallite-bearing grayish sylvite layer, the homogenization temperatures increase gradually in order of 23.8℃→27.7℃→38.6℃. This trend is consistent with the crystallization sequence of salt minerals, i.e., halite→sylvite→carnallite. Therefore, the evaporative intensity for the carnallite-bearing upper sylvite layer is obviously higher than that of the lower sylvite layer and the halite layer, implying that the formation of potash deposit might have resulted from the continuous concentration and salinization of the lake brine, and this salinization trend was probably caused by the rise of the brine temperature in the lake. Moreover, the change of brine temperature was influenced by atmospheric temperature. Therefore, the homogenization temperature of primary fluid inclusions in different layers of the deposit can not only reflect the ancient water temperature directly during the formation of the deposit but also indicate environmental atmospheric temperature of that time.