Based on Measures for Determining Limit Water Level（Discharge）in Drought, this paper takes downstream reaches of Linshui river as a case to analyze and calculate the water demand of main factors like urban - rural water supply, enterprise production, agricultural irrigation, shipping transportation, ecological environment and so on, by analyzing the historical data of hydrological station in Luojia village and combining with the investigation of water use types in river reaches. Finally, it determines the drought warning flow and drought warning water level of the control section of hydrological station in Loujia village.