The combination of ions of formation water from the Members 2 and 4 of the Upper Triassic Xujiahe Formation in Xinchang area, Western Sichuan depression, show the characteristics of water from marine strata, especially in the Cl-—Na+—Br-system, the water also show the mixture by brominerich compacting fluids from the adjacent shale and coal beds, and the enrichment of bromine can be used as a symbol of ballast water discharged, the modern formation water have been resulted from mixture of brominerich compacting fluids and water of marine. Because the connate water was flushed out of the sediments by compaction processes and replaced subsequently by younger compacting fluids and sea water，the values of the most samples significantly increased, especially for the water of the Member 4,the higher δ18O value indicate the more intensive water—rock reactions compared to the Member 2. Although Cl-, Na+, Br-concentration appears to be controlled by simple mixing rather than by diagenetic reaction, variation in K+,Ca2+,Mg2+,Fe2+,HCO-3and SO2-4suggest diagenetic modification of both formation waters. The diagenetic reaction assumptions that is proposed for the variation above are in line with previous researches about the diagenesis. More open water—rock reaction system results in formation water of the Members 2 of the Xujiahe Formation being riched in Ca2+,Mg2+,Fe2+,but poor in K+ compared to that of the Members 4 of the Xujiahe Formation water , by which both formation water can be distinguished from each other.