Abstract: As a medium for hydrocarbon migration and accumulation, formation water plays an important role in oil and gas entrapment. In order to further understand the relationship between formation water and hydrocarbon accumulation, based on hydrodynamic system classification of Mesozoic formation in the hinterland of Junggar basin, the paper analyzes pressure, salinity, type and oil & gas-bearing properties of the formation water in the hydrodynamic system, and demonstrates geological significance of the formation water by combining with the analyses on formation water origin and its genetic mechanism. The results show that 3 hydrodynamic systems are developed vertically in the Mesozoic strata; the formation water can be divided into 10 subclasses of 3 types. NaHCO3-type formation water is mainly related to surface fresh water, compaction released water of shales and hydrocarbon generation of organic matter; Na2SO4-type formation water mainly originates from infiltration of saline karst water and hydrocarbon generation and expulsion of coal-measure source rocks; CaCl2-type and MgCl2-type formation water are controlled by forward concentration and cation exchange. The differences of hydrocarbons with different formation water types depend on whether the hydrocarbon-bearing fluid is charged, charging sequence and duration and charging degree. For a series of fluid charging during geological processes, the earlier the hydrocarbon-bearing fluid charges， the longer the charging duration and the higher the charging degree is, the higher the hydrocarbon enrichment degree will be