In order to measure the comprehensive benefit of different water-saving measures in the process of green construction, a comprehensive benefit evaluation system was proposed based on which, a hierarchical model with 19 indicators from aspects of social, economic, ecological and technical benefits was established considering the field survey and related literature. Due to the complexity of the construction site environment, the entropy modified G2 method was adopted to weight the indicators. This solved the contradiction that a single weighting method could not simultaneously reflect the intention of the decision maker and the information contained in the data. In view of the extreme situation that there were only a small amount of sample data on the construction site, the TOPSIS analysis method was selected to quickly and accurately evaluate the comprehensive benefits of water-saving measures adopted on the construction site. Finally, a construction project in northwest China was selected as the verification case. The field results were compared with those of the entropy weighted TOPSIS evaluation model, which verified the reliability and feasibility of the model.