The Mianhuakeng deposit is the largest granite-related hydrothermal-vein-type uranium deposit in South China and Jurassic biotite granite serves as its primary host rock. However, the hydrothermal alteration and its constraints on uranium mineralization in this deposit are still poorly understood. To address these issues, we present a detailed study on the petrography, whole-rock geochemistry, and mineral chemistry of altered rocks and ores. Based on the field geological characteristics, these hydrothermal alteration halos can be classified into three alteration zones (zones 1 to 3) and one mineralization zone. Alteration zone 1 is the furthest away from the ore body and consists of extensive illitization and minor chloritization, whereas alteration zone 2 has partial albitization and dequartzification. The pervasive illitization, chloritization, and hematitization are considerable in the alteration zone 3 closest to the ore body. The mineralized zone exhibits widespread silicification, hematitization, and local chloritization. The mineralogical properties of chlorite indicate that at least two stages of hydrothermal alterations occurred within the alteration halos. The first one is the pre-ore hydrothermal alter-ation, which can be found in alteration zone 1. A weakly alkaline, Fe-rich, and Mg-poor fluid gave rise to this stage of hydrothermal alteration. The second one, known as the syn-ore hydrothermal alteration, appears in the mineralized zone that was created by an acidic, Mg-rich, Fe-poor fluid that overprints on alteration zones 2 and 3. The formation temperatures of chlorite from these two stages of hydrothermal alterations are 350-250 degrees C and 250-150 degrees C, respectively. Alterations and uranium mineralization are inextricably linked in the Mianhuakeng deposit. Pre-ore hydrothermal alterations improve the efficiency of fluid circulation and the proportion of ura-nium leaching from uraninite in granite. Uraninite in granites releases removable uranium through hydrothermal alterations to serve as a major source for mineralization. Syn-ore hydrothermal alterations are responsible for the redox reaction between U6+ in ore-forming fluids and Fe2+ released from reequilibration of Fe/Mg-rich chlorite, which results in uraninite (UO2) precipitation from hydrothermal fluids. Here, we suggest that alteration zone 1 with widespread illitization and uranium loss should be considered as the first-line vector for further uranium exploration in the Mianhuakeng deposit, and alteration zone 3 with significant chloritization and hematitization should be the crucial vector.