We divide the ecological functions of farmland into five types: soil conservation, water conservation, air purification, maintenance of biological diversity, entertainment and culture. Using the direct method of market evaluation, we establish the assessment model, to calculate the ecological service value of various functions, respectively, and then calculate the total loss of ecological value in farmland. Taking the case of Shaanxi Province, we calculate the total loss of ecological value in farmland in Shaanxi Province during the period 2000-2009 at 6366.3653 million yuan. Finally in order to rationally protect farmland, we put forth the following recommendations: correctly understanding the ecological benefit of farmland, and scientifically assessing the value of farmland; optimizing the industrial structure, and promoting intensive use of farmland; taking actions that suit local circumstances, and conducting rational planning of farmland use; strengthening multilateral cooperation, and establishing the common protection accountability mechanism of farmland.