This study preformed the analysis of phases, types, geochemical characteristics and genetic mechanism of carbonate cements from the sandstone reservoirs in the deep water area of Baiyun Sag, Pearl River Mouth Basin, South China Sea, based on identification and quantitative statistics of thin sections under microscope, analysis of cathodoluminescence and scanning electron microscopy, electron microprobe, measurement of carbon, oxygen and strontium isotope and homogenization temperature of authigenic fluid inclusions. The sandstone reservoirs consist of the Miocene Zhujiang Group and the Oligocene Zhuhai Group. Research result shows that three phases of the carbonate cementation occurred both in the Zhujiang and Zhuhai sandstones. The early carbonate cement of the Zhujiang Formation consists mainly of calcite and is characterized by high Ca and low Fe and Mg. Its isotopoic compositions（δ13CPDB：-243‰～029‰，δ18OPDB：-979‰～-308‰，87Sr/86Sr：07084～07109） show some differences from that （δ13CPDB：-937‰～-813‰，δ18OPDB：-711‰～-709‰，87Sr/86Sr：07138～07142）of the Zhuhai Formation, with the former resulting from precipitation of the carbonate supersaturated alkaline sea water in the shallow burial stage, and the latter related to the carbonate supersaturated alkaline fresh water. The interim carbonate is mainly ferrocalcite with characteristic of higher Ca, high Fe, and low Mg, and wide ranges of C and Sr isotope（δ13CPDB：-2088‰～-529‰, δ18OPDB：-111‰～-899‰, 87Sr/86Sr：07093～07151）. Chemical and geochemical features show that part of carbon was from CO2 released by decarboxylation of organic acid, and the other part （δ13CPDB：-538‰～-529‰） may be related to the substances from the deep crust. Ca2+, Mg2+, and Fe2+ ions necessary for the interim carbonate cements were derived from dissolution of feldspars and other detrital minerals in sediments, transformation of clay minerals, and deep thermal fluids. The late carbonate cements consisting mainly of ankerite（δ13CPDB：-283‰～-183‰, δ18OPDB：-945‰～-577‰, 87Sr/86Sr：07101～07162）are characterized by higher Fe and Mg contents, and relatively low Ca content, which are basically consistent with that of homochronous seawater. But their 87Sr/86Sr values are lower than that formed in a normal diagenetic evolution. It can be inferred that the formation of the late carbonate cements may be related to the dissolution and reprecipitation of the early carbonate cements and the bioclastic limestones, and likely affected by the deep hydrothermal fluids.