Pyrite is well developed in southeastern Ordos Basin, and occurs in anhydrite dissolved pores and karst caves, or is accompanied by anhydrite layers as dispersed grains or assemblages. The δ³⁴S values of the pyrite vary in the range of 10.50‰~24.00‰, 17.33‰ on average, indicating its thermochemical sulfate reduction (TSR) origin. The basic condition for TSR is thin layer of anhydrite, high temperature and abundant hydrocarbon, which is all qualified in southeastern Ordos Basin. In combination with its modes of occurrence, the well-developed pyrite accompanied with anhydrite dissolution is considered to be the product of TSR. The formation with the existence of pyrite has high content of Fe, ranging from 3 387.50×10^-6 to 23 112.50×10^-6, averagely 13 233.33×10^-6, which provided material source for the formation of pyrite. However, the H₂S concentration is low in the Ordovician carbonate, which was probably caused by the precipitation of pyrite.