Abstract: Human activities have dramatically changed the subsurface flow and caused problems such as salinization and desertification that degrade groundwater-dependent the ecosystem. Taking the shallow groundwater layer as the link, a conceptual model of phreatic water evaporation in arid plains is established. The model analyses the groundwater depth conditions of desertification and salinization in arid areas, and calculates threshold water table required for preventing these problems. The Luocheng Irrigation District at the junction of the middle and lower reaches of Heihe River is taken as an example to study the relation between groundwater depths and ecosystems patterns. The contributions of this study include: ① Describing the soil water movement principle caused by desertification outside the salinized oasis in the arid area, based on which, the method for calculating the threshold groundwater depth for preventing salinization is provided. By calculation, the critical groundwater depth leading to soil salinization is 1.3~1.5 m and the calculated critical groundwater level is 8~13 m; ② Discussing the appropriate values of soil equivalent pore diameter and liquid surface tension used for estimating the maximum rising height of capillary water; ③ Proposing a scheme to regulate the groundwater level regulation via controlling irrigation in the area to protect groundwater-dependent ecosystems while satisfying water demand for irrigation and other purposes.