Abstract：Deep-seated fluid activity and the recently proposed “exudative” metallogenic model for sandstone-type uranium deposits are closely associated with reducing geological environments. Therefore, a systematic understanding of uranium speciation, mobility, and controlling mechanisms under reducing conditions is essential for clarifying uranium transport in deep fluids and for improving genetic interpretations of uranium mineralization. In this study, the occurrence forms and migration potential of uranium in several representative reducing systems enriched in organic ligands, silica, sulfur, halogens, and carbonate are comprehensively reviewed. The results indicate that uranium in reducing environments does not necessarily occur exclusively as insoluble mineral phases. Instead, its speciation is strongly controlled by pH, redox conditions, ligand composition, ligand concentration, temperature, and colloidal interactions. Under acidic reducing conditions, uranium may occur as U(IV) -SO₄^2- complexes, U(IV) -F^- complexes, U(IV) -Cl^- complexes, and U(IV) -organic ligand complexes. In contrast, under neutral to alkaline reducing conditions, uranium is more likely to exist as uranyl carbonate complexes, Ca²⁺-U(VI) -CO₃^2- complexes, U(IV) -Si colloids, UO₂ colloids, or as uranium adsorbed onto organic matter-related carriers. The mobility of these uranium species varies markedly among different geochemical systems. Under ambient geological conditions, uranyl carbonate complexes, Ca²⁺-U(VI) - CO₃^2- complexes, UF₃⁺, UF₃²⁺ , UF₄⁰ (aq), U(IV)-Cit₃^3- complexes, and U(IV)-Si colloids may exhibit relatively strong mobility. Among them, carbonate and calcium-uranyl-carbonate complexes can significantly enhance uranium solubility under weakly alkaline conditions, whereas U(IV)-Si colloids may remain dispersed and mobile owing to their nanoscale particle size and favorable surface charge characteristics. Under high-temperature geological conditions, UCl04 (aq) may become an important mobile uranium species, particularly in acidic, high-salinity reducing fluids. Ligand concentration is a key factor governing the stability and migration capacity of complexed uranium species, and it also influences the formation and persistence of U(IV)-Si colloids. Meanwhile, positively charged mineral colloids may restrict the migration of U(IV) -Si colloids through colloid-colloid interactions, thereby reducing uranium mobility. The effect of temperature on uranium speciation and mobility has been investigated only in a limited number of systems, and its role remains insufficiently constrained. Overall, previous studies have mainly focused on room-temperature and atmospheric-pressure systems, or on single hightemperature experimental conditions. However, the geochemical behavior of uranium species under coupled lowto medium-temperature and medium- to high-pressure conditions, as well as under medium- to high-temperature and medium- to high-pressure conditions, remains poorly understood. Future research should therefore emphasize laboratory simulations of reducing environments under low- to medium-temperature and medium- to highpressure conditions, and progressively extend such investigations to medium- to high-temperature and mediumto high-pressure systems. These efforts will provide important constraints on the occurrence forms, migration mechanisms, and evolutionary behavior of uranium species in deep reducing ore-forming environments.