Tin (Sn) isotope geochemistry has great potential in tracing geological processes. However, lack of equilibrium Sn isotope fractionation factors of various Sn species limits the development of Sn isotope geochemistry. Equilibrium Sn isotope fractionation factors (124Sn/116Sn and 122Sn/116Sn) among various Sn(II, IV) complexes in aqueous solution were calculated using first-principles calculations. The results show that the oxidation states and the change of Sn(II, IV) species in hydrothermal fluids are the main factors leading to tin isotope fractionation in hydrothermal systems. For the Sn(IV) complexes, Sn isotope fractionation factors depend on the number of H2O molecules. For the Sn(II) complexes, the Sn isotope fractionation between Sn(II)?F, Sn(II)?Cl and Sn(II)?OH complexes is mainly affected by the bond length and the coordination number of anion, whereas the difference in 1000lnβ values of Sn(II)?SO4 and Sn(II)?CO3 complexes is insignificant with the change of anion coordination number. By comparing the 1000lnβ values of all Sn(II, IV) complexes, the enrichment trend in heavy Sn isotopes is Sn(IV) complexes > Sn(II) complexes. The equilibrium Sn isotopic fractionation factors enhance our understanding of the tin transportation and enrichment processes in hydrothermal systems.