Abstract: Based on the statistics and analysis of the test results of formation water samples of He 8th Member of Tianhuan depression, Ordos basin, the geochemical characteristics of the northern and southern parts of the area are studied, and the differences were compared. The results show that the formation water of He 8th member of Tianhuan Depression is acidic and CaCl₂ water type, and the mineralization degree, Cl⁻ content and concentration in the area are higher in the north than in the south. It reflects that the sealing condition of formation water in the Tiekesumiao area in the north is better than that in Qingshimao - Lizhuangzi area in the south, which is more conducive to the accumulation and preservation of gas reservoirs. The Geo-chemical characteristic parameters of formation water in the north and south have good correspondence with the formation conditions of the gas reservoir. The formation water in the northern area is characterized by low sodium-chloride coefficient, low desulfurization coefficient, low sodium-calcium coefficient and high metamorphic coefficient. The formation water is of Ⅳ-type and Ⅴ-type original formation water, which is favorable for the accumulation and preservation of gas reservoirs. In the southern area, the formation water of Ⅱ-type, Ⅲ-type and Ⅳ-type shows poor sealing conditions, and the formation water communicates with the outside world, which is not conducive to the accumulation and preservation of gas reservoirs.