Based on experimental data and theoretic calculation results from the literatures, this paper has summarized the valences and geochemical behaviors of uranium in melts and redox and other conditions of uranium being dissolved into fluids. Then the cause for the large difference in ages of uranium deposits and host granites, the source of uranium and the origin of hydrothermal fluids have been elucidated in this paper. The oxygen fugacity of either mantlederived melts or granitic magma is below that of the magnetite—hematite (MH) buffer, while the oxygen fugacity for uranyl ion (U6+) stable in fluids is much higher than MH. This implying that neither mantlederived melts nor granitic magma could reach the oxygen fugacity for U6+. Uranium in magmas occurs as uranous ion (U4+) which enters crystals of uraninite and/or other accessory minerals in later stage of magma evolution and hardly enters magmaderived fluids unless under Frich alkaline magma condition. This is why there is almost no uranium deposit formed by granitederived fluids. As uranium in fluids occurs as uranyl ion or complexes, it is the key process for forming granitetype uranium deposits that hydrothermal fluids with high oxygen fugacity ultimately originated from meteoric water leach uranium from Urich granites. As for the granitetype uranium deposits in South China, Indosinian Urich prealuminous leucogranites were the uranium source rocks, and late Yanshanian tectonic extension and dike magmatism provided the heat and the fissure system in granites for meteoric water infiltrating and cycling to leach uranium from the granites. Such fluids became Urich hydrothermal fluids and finally formed the uranium deposits.