Groundwater is the main source of water consumption of natural vegetation in arid regions. It is an effective approach to study ecological water demand of natural vegetation by phreatic evaporation. In order to study the ecological water demand of Tarim river basin, based on the observation data of phreatic evaporation on bare lands at the Aksu Water Balance Experimental station from 1989 to 1996, by analyzing the relationship of phreatic evaporation, depth of phreatic surface and evaporation of water, taking the limit rate of phreatic evaporation as the control condition, and based on the objective law that the relation between phreatic evaporation and evaporation of water is nonlinear, we establish models for calculating phreatic evaporation on bare land, which can fully reflect the law of phreatic evaporation in the Tarim river basin. According to the data of depth of phreatic surface and soil moisture when pheratic level decline is caused just by evapotran-spiration on Tamarix-vegetated land from 2003 to 2004, we calculate the amount of phreatic evaporation and set up models for calculating phreatic evaporation on Tamarix- vegetated land. Phreatic evaporation on bare land and Tamarix-vegetated land could be transformed each other by a Tamarix vegetation conversion coefficient. The test results show that the calculation accuracy of the models is high and the models are suitable for Tarim river basin. Groundwater is the main source of water consumption of natural vegetation in arid regions. It is an effective approach to study ecological water demand of natural vegetation by phreatic evaporation. In order to study the ecological water demand of Tarim river basin, based on the observation data of phreatic evaporation on bare lands at the Aksu Water Balance Experimental station from 1989 to 1996, by analyzing the relationship of phreatic evaporation, depth of phreatic surface and evaporation of water, taking the limit rate of phreatic evaporation as the control condition, and based on the objective law that the relation between phreatic evaporation and evaporation of water is nonlinear, we establish models for calculating phreatic evaporation on bare land, which can fully reflect the law of phreatic evaporation in the Tarim river basin. According to the data of depth of phreatic surface and soil moisture when pheratic level decline is caused just by evapotran-spiration on Tamarix-vege tated land from 2003 to 2004, we calculate the amount of phreatic evaporation and set up models for calculating phreatic evaporation on Tamarix- vegetated land. Phreatic evaporation on bare land and Tamarix-vegetated land could be transformed each other by a Tamarix vegetation conversion coefficient. The test results show that the calculation accuracy of the models is high and the models are suitable for Tarim river basin.