Abstract: Formation water is the groundwater that coexists with oil and gas and has important relation with hydrcarbon generation, migration and accumulation. Based on chemical analysis of formation water in the middle Yangtze region, the statistical analysis of reservoirs formation water salinity has been carried out, the water chemical characteristics factor, including metamorphic coefficient, desulfurization coefficient, etc have been calculated. The analysis results show the formation water in the middle Yangtze region is in alternating block and alternate stagnation zone, and oil and gas is preserved well. Combination of the formation water mostly belongs to the free exchange zone and alternate block belt, but the oil and gas is poorly preserved. The area in the block near eastern Chongqing and western Hubei is the most favorable for oil-gas preservation, followed by Jianghan plain, and the area western Hunan is the worst.