Abundant ankerite occurs in the Tertiary reservoir of Baiyun Sag, Pearl River Mouth Basin. Influenced by heat flow background in the study area, ankerite is mainly distributed in the high geothermal gradient area with shallow- medium burial depths from 1000 to 3000 m and temperature between 60~130℃. The δ13C value (-2.83‰~-1.83‰) of the ankerite, which is consistent with that of magmatic origin CO2 in the gas pool, indicates its inorganic origin. The values (-9.45‰~-5.37‰) of δ18O and 87Sr/86Sr (0.7101~0.7161)also show that precipitation of the ankerite was affected by the deep hydrothermal fluid activity. Case study of two typical CO2 gas wells shows that the homogenization temperature of fluid inclusions and evolution of montmorillonite to illite prove that the reservoir underwent thermal fluid invasion. CO2 thermal fluid activity regions are not only the concentrated distribution area of ankerite but also the places hosting typical authigenic mineral assemblage of dawsonite+ankerite+siderite, which can trace CO2 migration and accumulation. The contents of ankerite and kaolinite in the formation are positively correlated with that of CO2 in the gas pool, and the content of chlorite is negatively correlated with that of CO2 and ankerite. Aabundant CO2 hydrothermal activity was the main reason for the precipitation of developed ankerite in the high geothermal gradient area. CO2 of inorganic magmatic origin was the main carbon source for precipitation of the ankerite. The abnormal transformation of clay minerals (such as chlorite) caused by CO2 thermal fluid charging provided the necessary Fe2+, Mg2+ ions for precipitation of the ankerite.