Sandstone type uranium deposit is one of the most important type of uranium mineralization in the world, which is generally dominated by redox mineralization of supergene fluids. Although traces of hydrothermal fluid activity were found in some sandstone type uranium deposits, the relationship between the hydrothermal fluids and uranium mineralization is still unclear. In this study, the newly discovered uranium mineralization in the sandy conglomerate of the lower Bayingebi Formation of Lower Cretaceous in the Bayingebi basin is selected as the research object. The petrology, mineralogy and geochemistry of uranium ore were studied by means of microscopic identification, electron microprobe (EPMA), uranium content, uranium valence and trace element analysis.The results show that uranium mineralization occurs in the purplish red sandstone in the lower Bayingebi Formation, which is related to unconformity interface and secondary faults. Uranium is dispersed in collophanite and associated with metal sulfides such as galena and sphalerite. The Sr, Y, Mo, W and REE are significantly enriched in the ore. All these indicate that the formation of ore is closely related to deep fluids.Our research shows that the volcanic hydrothermal solution formed by the eruption of the Suhongtu Formation basalt mixes with the surface atmospheric precipitation to form a weak acidic oxidizing fluid during the rising process. The fluid moves upward along the unconformity and fault and continuously extracts U and P from the surrounding rocks. When it meets the reducing substances such as siderite in the overlying Bayingobi Formation glutenite, it forms the acid- alkali and redox contact interface, which then induces the precipitation of uranium and phosphorus.The uranium mineralization found in the new horizon widens the prospecting space and direction of uranium exploration in Bayingebi basin.