Carbon-, oxygen- and strontium-isotope studies have been performed on hydrothermal calcites from the Xianghualing tin-polymetallic deposit to investigate the sources of ore-forming fluids and the mechanism of mineralization. The results show that there are light carbon (-5.4‰ ~ -1.4‰ V-PDB) and oxygen isotope compositions (+6.1‰ ~ +13.9‰ V-SMOW) and higher strontium isotope compositions (0.7101 ~ 0.7230) for the calcites formed in the early stage of mineralization, while for the calcites formed in late stage of mineralization, the δ13CPDB, δ18OSMOW and 87Sr/86Sr values are +0.2‰ ~ +0.6‰, +19.4‰ ~ +21.5‰ and 0.7101 respectively. It is suggested that the calcites formed in the early stage of mineralization resulted from the water/rock interaction of magmatic hydrothermal fluid and marine carbonate, and the carbon in ore-forming fluids are mainly derived from magmatic fluids and marine carbonates. It is revealed from simulation calculation that the calcites formed in the early stage of mineralization resulted from CO2 degassing where 5 to 10 percent of CO2 was lost. In contrast, the calcites precipitated in late stage of mineralization may be formed by leaching marine carbonate host rock in this area at lower temperatures, and the carbon in ore-forming fluids are mainly derived from the marine carbonate rocks. Strontium in the ore-forming fluid is mainly of granite and marine carbonate rock origin and there is a decrease in the proportion of strontium originated from granite and an increase for the strontium of marine carbonate origin during mineralization.