The No. 512 Deposit, which is an interlayered oxidation zone type sandstone uranium deposit in the Yli basin, is the first acid in-situ leaching uranium deposit of industrial significance in China. The authors briefly introduce both its geological characteristics and the in-situ leach techniques, and quantitatively study the geochemical activity of uranium during in-situ leaching by both the new Gibbs free energies and equilibrium constants of uranyl complexes. Firstly, both calculated E0 and Eh indicate that H2O2, nitrate and the free oxygen in air all can increase the Eh in aqueous solutions and can oxidize the U4+ in mineral phases. Free oxygen in air is considered to be an economical oxidant for uranium deposits, whose U4+/U6+ ratios are comparatively low. Secondly, the formulas for calculating uranium species in both underground water and solution from sulphuric acid in-situ leaching are deducted by chemical equilibrium calculation, and the calculated results show that: (1) the uranium mostly exists in the form of uranyl carbonate complexes, mainly in the form of uranyl tricarbonate and uranyl dicarbonate anions in the ore-bearing aquifers of the No. 512 deposit, in which UO2CO3 is from 3. 80% down to 0. 06%, UO2(CO3)22- from 21. 91% down to 3. 12%, but UO2 (CO3)34- from 74. 28% up to"96. 82% along with pH from 7. 31 up to 8. 20; and (2) the uranium mostly exists in the form of uranyl sulfate complexes in the solution from the No. 512 deposit, in which UO2SO4 is 63. 28% -63. 86% ,UO2(SO4)22-16. 55%-21. 36% ,UO22+15. 04%-19. 62% , and the formula indicates that the ratio of uranium species varies with both the pH and the total SO42- contents in the solution and their relationship is nonlinear. In order to have the best proportions of uranium species in the solution for increasing the adsorptive capability of ion-exchange resins, the pH and total SO42- contents in working solution may be adjusted by the formulas in this paper. Finally, it is suggested to use the new exact thermodynamic data for the calculation because thermodynamic data will affect the calculation of chemical equilibrium by an exponential factor.