Abstract: The pressure system in the central depression of the Songliao Basin was analyzed systematically. A large amount of data about formation water was studied and compared, based on which the ions pattern as well as the diagenesis reaction of formation water were discussed. It has been concluded that abnormal pressure obviously influences geofluid and diagenesis. Both overpressure and underpressure can increase TDS; however, their influencing mechanisms are different. Water type varies in different pressure systems, leading to different diagenesis effects. In overpressure and normal pressure systems, NaHCO₃ and NaCl type water is dominant, and the dissolution of minerals works as the main diagenesis effect. As to underpressure system, CaCl₂ type water is dominant, and the main diagenesis effect is the albitiztion of feldspar.