Based on the natural and observed runoff data at Linyi station of Yi River, conforming to the basic principle of water balance, and according to the ecological water demand and water withdrawals of the river, a kind of ecological water demand analysis method was put forward to analyze the water withdrawals, water withdrawals ratio and guarantee degree of ecological water demand of Yi River under the condition of different level years on the premise of meeting the ecological water demand requirements. The results show that the thresholds of ecological water demand, ecological water demand ratio are 0. 343 billion~2. 063 billion cubic meters and 12. 50%~75. 17%, respectively. The thresholds of water withdrawals, water withdrawals ratio are 0. 680 billion~2. 400 billion cubic meters and 24. 79%~87. 50%, respectively. The actual water withdrawals, actual water withdrawals ratio are 0. 865 billion cubic meters and 31. 53%, respectively. Under the condition of wet year, normal year, dry year and especially dry year, only the minimum ecological water demand of some months in especially dry year can’t be met, and the guarantee degree of optimal ecological water demand of Yi River is relatively at a low level. Therefore, while reducing the water withdrawals from the river, it is necessary to increase modest ecological water by water scheduling of certain water conservancy project, keeping the ecological environment of Yi River healthy and stable.