The social systems risk flood plain in downstream of Yarkant River has increased in recent years,so EWS was set up on the Yarkant river. From the warning process of EWS automatic monitoring stations on Yarkant River, EWS is an effective measure to reduce the risk of glacial lake outburst floods. Combined historical data and economic losses resulted from flood,the proposed a method to determine the warning parameters of EWS.The method has certain accuracy. However ,the current vulnerability of social system in Yarkant River flood plain must also be taken into account so as to determine reasonable warning parameters.