The reducible sulfur associated ore was brought into soil in many ways, which will further acidify its surrounding area. In the Huanjiang River of Guangxi, more than 600 hm2 of farmland have been acidified, so the formation of soil acidification and the distribution characteristics of acidity were studied. The chemical analysis results showed that soil acidification came from oxygenation of reducible sulfur. Based on the results from mineral (X-ray), sulfur speciation (EXAFS) and Pb isotope analysis, the cause for the formation of soil acidification can be found out: In 2001, the flood destroyed the tailing dam located in the upstream of the Huanjiang River, and carried the tailing into farmland, so the reducible sulfur in tailings was oxygenated into H+ after a certain period of time. The area on both banks of the Huanjiang River was all affected by flooding, which should be given more attention. The spatial variabilities of pH, St, SO42- (Ss), TAA, TPA and Fe in the investigated area were very large, but the contents of acidity, iron and sulfur in flooded soil were much higher than those in the unflooded soil. The soil near mining sites was not acidified because of secondary pollution by alkaline tailings, but has potential acidification risk; while the soil in mid- and down-stream of the river had been acidified more or less, and most soil still has potential acidification probability. It is supposed that acid pollution was more serious in dry soil than in paddy soil, so converting dry land into paddy land could decrease the oxidative rate of reducible sulfur.