During social transition and with the deepening of reform and opening-up，more and more young parents in the rural areas go out to work in cities ,leaving their young children to live with grandparents.Based on the survey in L Village,Xishui county,this paper discovers that role of left-behind grandmothers in bringing up grandchildren has changed.Such changes have led to the transformation in rural family structure and familiy relationship,including the formation of a new triangle family between grandparents and grandchildren and a core family which crosses the geneations; intimate relationships between grandparents and grandchildren and improvement of relationship between mothers-in-law and daughters-in-law.Altough the re-undertaking of roles for rural left-behind old women as second mother enables young parents to go out to work in the cities without worries,the raising for children is still negatively influenced.Therefore,this paper proposes several corresponding suggestions.