Abstract: XY-1 well is a shale gas well of Lianyuan sag in Xiangzhong depression.During the production process,the formation water changed from calcium chloride type to sodium bicarbonate type.To find out the change reasons,through analysis of hydrocarbon preservation conditions and formation water chemical characteristics,the formation water salinity,chlorine coefficient magnesium,sodium and chlorine coefficient,desulfurization coefficient,carbonate equilibrium coefficient are small,with good hydrocarbon preservation conditions.Water-type change of formation water may be mainly affected by the injection of CO2 instead of fresh surface water.