Abstract: 【Objective】It is important to quantify the water conservation services of the farmland ecosystem scientifically and accurately, which is helpful to play roles of regulating water and stabling the water supply, alleviating the water scarcity crisis.【Method】Under 3S, the quantity of water conservation in the farmland ecosystem in Guanzhong-Tianshui economic zone was estimated by the method of soil water storage, and the value with Shadow Engineering and Cost Accounting Methods.【Result and conclusion】During the 30 years from 1980 to 2010, the change tendency of the quantity and value of water conservation in the farmland ecosystem in Guanzhong-Tianshui economic zone had been decreasing at the beginning and then increaseing. From 1980 to 2005, the water conservation quantity decreased from 3.221 9×10⁹ m³·year^-1 to 1.969 1×10⁹ m³·year^-1, and water conservation values decreased from 2.158 7×10⁹ yuan·year^-1 to 1.319 3×10⁹ yuan·year^-1, with an average annual decrement of 1.56%; while from 2005 to 2010, the water conservation quantity increased from 1.969 1×10⁹ t·year^-1 to 3.163 8×10⁹ t·year^-1, and water conservation values increased from 1.319 3 × 10⁹ yuan·year^-1 to 3.221 9×10⁹ yuan·year^-1, with an average annual increment of 7.42%. Under the positive guidance of returning farmland to forest/grass land policy, even if the farmland area was reducing gradually, there was an increasing trend of the quantity and value of water conservation in the farmland ecosystem in Guanzhong-Tianshui economic zone. And there was a certain temporal and spatial variation: It decreased at the beginning and then increased in the time; while in the space, it was high in in the southeast plains and low in the northwest mountainous areas.