Based on the relationship between uranium minerals and pyrite from different types of uranium ores in the Mianhuakeng deposit, the authors investigated the mechanism for the deposition and enrichment of uranium during the ore-forming process. The research shows that uranium orebodies in the Mianhuakeng deposit generally occur in vein-filling or vein-disseminated form. Uranium ore types include fluorite type, carbonate type, siliceous vein type and reddening type. Although different ore types have different gangue minerals formed in the mineralization period, they show common characteristics as follows: Uranium minerals in the central part or along the vein walls are alternatively arranged with gangue minerals or occur in veinlike or disseminated form in cataclastic altered granite and are inlaid with gangue minerals such as calcite, fluorite and microcrystalline quartz. Uranium minerals and pyrite are closely associated with each other in aggregate or relatively independent states, and the boundaries between them and gangue minerals in the mineralization period are straight, with good crystal type and in inlaid growth form. The aforementioned evidence indicates that there is no sequential generative relationship between pyrite and uranium minerals, and that they are all co-crystalline products of ore-forming fluid. Uranium precipitation had nothing to do with redox reaction. In contrast, decrease of pressure and temperature and the change of pH and solubility were the main factors that triggered the precipitation of uranium and gangue minerals.