At present, due to the diversity of environmental conditions on the construction site, the comprehensive benefits arising from different water-saving measures are very different. Nowadays, by combining with local investigations and Delphi method, we finally find out 19 comprehensive benefit evaluation indexes and set up the comprehensive benefit evaluation index system. Through the introduction of small sample model of TOPSIS-The Comprehensive Method of Entropy proportion, the comprehensive benefit of each water-saving measure is evaluated. Using this method can not only reduce the influence of the subjective evaluation results, but also solve the hard problem of selecting the high efficiency water saving measures which are suitable for the construction environment under the condition of small sample data. Finally, the comprehensive benefit evaluation in the construction process of a large construction project in Gansu Province is taken as the research object, which proves that the designed evaluation index and method are scientific and reasonable, the evaluation result is objective and accurate and has the significance of popularization.