The Simao basin is currently the sole sedimentary basin which hosts ancient solid potash deposit in China. But it forming time, provenance features and transgressive direction have long been controversial. Based on formation conditions, occurrence forms and isotope fractionation mechanism of marine anhydrite, this paper analyzed the sulfur isotope geochemical characteristics of evaporite in the well MZK- 3 in detail. The results are as follows: (1) anhydrite in halite can be formed from the precipitation of salt in evaporite basins. Independent stratiform anhydrite can be formed from the dehydration of original deposited gypsum due to high temperature from deep burial. (2) The sulfur isotope of anhydrite in halite is characterized by “double peaks”, with δ34S values of 14‰~16‰ and 8‰~10‰ or 6‰~8‰, respectively. This may reflect the dual sources of sulphate: one from seawater and the other from fresh water or volcanic hydrothermal fluid. (3) δ34 S values of the stratiform anhydrite can be contrasted regionally. 87Sr/86Sr values of the samples may represent a marine environment. (4) δ34 S values of the stratiform anhydrite decease from south to north, reflecting that relative influence of fresh water or hydrothermal fluid on evaporite basin correspondingly increases from south to north, and thus imply the transgression direction. Therefore, the study of anhydrite not only reveals source, sedimentary environment and trangressive direction, but also provides a useful reference for investigation of potash deposits in the study area.