Due to its extremely vulnerable ecological condition and its seriously degraded vegetation, the Ejina Oasis, located in lower reaches of the Heihe River, has key importance for realizing the goal of improving the environmental quality of the whole Heihe River Basin. The present plan for allocating the water to areas along the river is characterized by definite yearly quantity and inflow-determined letting-out time. Under this scheme, the water requirement by plant and water supply through allocating are often not simultaneous, the growing of the vegetation in the oasis is consequently affected. So, we should pay as same or more attention on the letting-out time of water as on water quantity allocated. In order to make the time of requirement and supply meet, we must be aware of the critical period of the plant species in the oasis. By monitoring the temporal-spatial variation of groundwater level at lower reaches of the Heihe River as well as analyzing the relationship between water consumption by natural vegetation and eco-hydrological process in the Ejina Oasis, we pointed out the key periods and quantity of ecological water requirement for the local vegetation as follows. At least 0.80×108 m3 and 1.08×108m3 of ecological water should be replenished in April and August, respectively, if rational regulating reservoirs outflow and channel runoff are concerned.