The distribution relationship between oil and water in Ahe Formation of Jurassic in the eastern Kuqa Depression, Tarim Basin is complicated, and the characteristics of formation water and its relationship with oil and gas reservoirs are lack of systematic study. In order to clarify the chemical characteristics of formation water and its relationship with reservoirs, based on the selection of formation water samples, the formation water test data of Ahe Formation in eastern Kuqa Depression are analyzed to study the distribution characteristics of water type, TDS and ionic ratio coefficients of formation water and its significance to reservoirs. The results show that the formation water of J₁a is mainly NaHCO₃ type,followed by CaCl₂ type and MgCl₂ type, and has relatively low salinity. The TDS and ionic ratio coefficients of formation water are controlled by the structure characteristics: In the open area of the structure and the pinch-out area of the formation, the formation water is mainly NaHCO₃ type with low salinity, the sodium chloride coefficient and desulfurization coefficient are high and the deterioration coefficient is negative. Nevertheless, the formation water in the closed formation has the opposite chemistry feature. There is a good coupling relationship between the chemical characteristics of formation water and the plane distribution characteristics of oil and water, with Dibei oilfield as an example, although YN4 and YS4 wells had experienced oil and gas filling, but the formed oil and gas escaped along the fault, and the chemical characteristics of the formation water and the kaolinite content reveal that the formation was affected by the meteoric water that entered along the fault. On the contrary, the chemical characteristics of formation water and kaolinite content in well YN2 indicate that the formation is well sealed and less affected by meteoric water, which corresponds to the distribution of oil layers. The relationship between the chemical characteristics of formation water and the distribution of oil and gas reservoirs provides a basis for subsequent exploration, development and evaluation of tight oil and gas reservoirs.