Abstract: Due to anthropogenic and long-term geological factors, radionuclide leakage may occur in deep geological repositories of radioactive waste when groundwater infiltrates a failed waste canister. The released radionuclides can migrate to near and far field regions through groundwater, posing a serious threat to the ecological environment and human health. As a consequence, knowledge of radionuclide speciation in groundwater is essential for understanding their transport behavior. Uranium is one of the key elements of significant concern for deep geological disposal of high-level radioactive waste. In this study, the uranium species in groundwater from boreholes BS03(pH=9.43) and BS05(pH=7.51) located at the Beishan area(Gansu province) were calculated by the hydrogeochemical software PHREEQC(version 3). The effects of pH, temperature, and initial uranium concentrations on ternary uranyl species are presented. The results indicate that UO₂CO₃(aq) and (UO₂)₂CO₃(OH)_3^- are the dominant species in a mildly acidic condition, while CaUO₂(CO₃)_3^2- and Ca₂UO₂(CO₃)₃(aq) are the major species in neutral and weakly alkaline conditions. In BS05 groundwater, the concentration of Ca²⁺(5.11 mmol/L) is about three times higher than that of Mg²⁺(1.58 mmol/L). However, the concentration of calcium uranyl carbonate is estimated to be about 250 times higher than that of magnesium uranyl carbonate(MgUO₂(CO₃) _3^2-). With an increase of temperature, Ca₂UO₂(CO₃)₃(aq), CaUO₂(CO₃)_3^2- and UO₂(CO₃)_3^4- concentrations progressively diminish. At uranium concentrations between 4×10⁻⁴ and 1×10⁻¹ mmol/L (25 ℃), the dominant uranium species, in order of decreasing concentration are: Ca₂UO₂(CO₃)₃(aq)(x＞62%)＞CaUO₂(CO₃)_3^2- (x≈35%)＞MgUO₂(CO₃)_3^2- /UO₂(CO₃)_3^4- (x≈1%), irrespective of the concentrations of Ca²⁺, Mg²⁺, and Na⁺. In the near-neutral and slightly alkaline conditions, the results suggest that the formation of neutral Ca₂UO₂(CO₃)₃(aq) and anionic CaUO₂(CO₃)_3^2- in Beishan groundwater reduces the distribution coefficient between uranium and environmental geo-medium, thereby enhancing the mobility and migration potential of uranium. The research findings contribute to understanding the transport and release mechanisms of uranium, as well as interactions between uranium species and engineered and natural barriers.