Water-saving irrigation is an effective way to solve the contradictions of the "water and field" in our country. The vast but there was a terrible shortage of water resources of western region, in order to improve the effective utilization of water resources, the construction of a large number of water-saving irrigation projects, but the post - evaluation work has not been carried out in a timely and effective way to find out the shortcomings of the irrigation projects and make an improvement. Taking the irrigation projects of Fenyang city, Shanxi Province from 2005 to 2013 as an example, conduct post-evaluation in accordance with national standards and norms. The grey relational degree method was used for post-evaluation on the basis of improving the post-evaluation index system and evaluation content. And the results pointed that in the system layer, the correlation degree of the construction implementation evaluation is the largest, followed by the effect evaluation. The correlation degree of the target evaluation and social impact assessment is relatively small, and the degree of reaching the expected target is not ideal, which needs to be further improved. And then, in the target layer, the correlation degree of process evaluation is the largest, which indicates that the process evaluation result of the project is better, and the reality is the closest to the expected target. The comprehensive evaluation has the smallest correlation degree, which indicates that the actual situation is far from the expected target and needs further improvement. In the index layer, the completion status of the index can be determined by the correlation coefficient, and then targeted improvement can be made, and scientific basis can be provided for the projects under construction and planned construction. At last, for this project, the overall evaluation result is good, but the project operation condition needs to be further improved.