The northeastern Ordos basin is an important metallogenic area for sandstonetype uranium deposits in China. Field observation and microscopic identification for the Zhiluo Formation orebearing strata in the Daying uranium deposit suggest that pyrite occurs as several form and there is a certain correspondence relation between their macroscopic and microscopic occurrence. Sulfur isotope analysis of pyrite and study of alteration minerals indicate that δ4S values vary from -36.5‰ to -27.3‰, with an average of -32.28‰, suggesting an obvious biogenic process. Meanwhile, the discovery of framboidal pyrite and uranium minerals in biological debris cells also confirms biological effects on uranium mineralization. U6+ and Fe2 + reacted with light H2 32S in the groundwater, resulting in the paragenesis of pyrite closely and uranium mineralization.