Abstract: This study developed and validated 1k antiferromagnetic (AFM) model for large-scale UO₂ surface calculations, investigating O₂ and H₂O adsorption and dissociation mechanisms. We theoretically confirmed experimental observations of chemisorption states for both O₂ and H₂O on UO₂(111), and systematically revealed adsorption and dissociation pathways on UO₂(111) and (110) surfaces. Employing ab-initio atomistic thermodynamics method, the adsorption thermodynamic phase diagrams under environmental conditions are constructed, demonstrating that O₂ monolayer adsorption originates from intermolecular electrostatic repulsion while H₂O multilayer adsorption arises from hydrogen bonding, leading to preferential H₂O adsorption and dissociation. H₂O dissociation introduces excess charges, promote subsequent O₂ adsorption in mixed environments. The adsorbed O₂ then facilitates OH dissociation, further reacting with H to form OH and O, ultimately establishing a water-oxygen cycle reaction mechanism: O₂+OH@UO₂ \rightarrow OH@UO_2+x. This work explains the reaction sequences observed in water-oxygen environments and provides key mechanistic insights into uranium oxidation corrosion.