It had been confirmed that there was high-content fluorine in topsoil (clay for coal-combustion) of Hehua Village， Zhijin County，Guizhou Province， which was considered to be closely associated with the long-term occurrence of endemic fluorosis. The new discovery in this article confirms that the mean value of fluorine content of clay used for coal -combustion in this village is 1284μg/g(620.8～2816μg/g，n = 45) ；The mean value of apparent acidity of topsoil （pHa）in this village is 4.92 (4.03～6.35，n = 45) and the mean value of actual acidity is 3.67(2.09～5.03，n = 45)；The measured average value of total sulfate radical was 4465μg/g（1165～7003μg/g，n = 45）. In addition, the apparent acidity and the content of total sulfate were positively correlated (correlation coefficient:0.7920). It could be concluded that clay for coal-combustion has been sulfated with a high fluorine content. Sulfuric acid hydrate(H2SO4·H2O) must present in the sulfated clay. Under certain circumstance，such as heating or burning， the prevalence of the acid(H2SO4·H2O) would decompose fluoride in the clay and produce volatile hydrogen fluoride(HF) with mean amount of estimated HF 455 μg/g（9.1～1643 μg/g，n = 45）.This paper putted forward that the clay would be sulfated and has a potential of releasing HF. It still remains to be discussed whether other endemic fluorosis areas have the same phenomenon.