Abstract: Vertical variation of formation water is not obvious，with brine characteristics of water type for the CaCl2，based on the 247 analysis samples of XiaoquanFenggu structural belt.95.2% strata water chemical is Cl-，Na+，Ca2+，Cl- and Na+ have strong relationship with the TDS.The main ions in content is much higher than sea water except Mg2+and SO2-4.The formation water is a typical hypometamorphism sedimentary water，and its formation is the result of concentrated salinity，a positive metamorphose.Studies show that the syndepositional formation water of the second member is seawater，and the fourth member is mixed water.NaCl and CaCl2 are the main presentation forms.Na+ deficit and abundant Ca2+relatively to the seawater is the basic feature，calcitization is obvious，and halite dissolution and evaporation is weak.The best explanation for this relationship is that albitzation of plagioclase controls the Na and Ca abundances.Rock water interaction indicates a net cation exchange rations of 2 Na+ for 1 Ca2+.In the study area desulfated elimination is not obvious.

Key words: geochemistry characteristic of formation water, origin analysis, water rock interaction, Xiaoquan Fenggu structural belt 