Geochemistry of formation waters in the Shiwu depression of the Songliao basin, northeast China, shows a distinct variation in different pressured systems. The study area is composed of terrigeneous clastics in fluvial-lacustrine environments, which contain formation water with salinity ranging from less than 1000 to 12,000mg/l. Water composition varies with depth and hydrochemical regions. In the underpressured strata deposited during the rifting period, formation water is characterized by water type of CaCl2 with higher salinity. NaHCO3-dominanted water with lower salinity occurs in the normally pressured strata deposited during the post-rifting period. In the transition area between the normally pressured and underpressured zones, total dissolved solid (TDS) content ranges from 1000 to 7600mg/l and increases with depth. In this halite-free basin, brackish water may be attributed to the evaporation of formation water and water–rock interaction. In the deep buried underpressured water, a predominated diagenesis reaction resulting in Ca and Cl enrichment and reduction of Na may be related to albitization of plagioclase following the basinal fluid line (BFL). The results of this study indicate that salinity variation of formation water and diagenesis reaction is closely related to hydrochemical environments within different pressured systems.