China’s water resources are extremely scarce. Statistics show that agricultural water consumption accounts for more than 60% of the total water consumption in the country every year, and farmland irrigation water accounts for the largest proportion, accounting for about 90% of agricultural water consumption. It can be seen that irrigation water is the key to water saving. Taking the south of Jiangsu Province as the research background, this paper constructs an evaluation index system of farmland irrigation water use efficiency from four dimensions of water consumption level, irrigation water saving, water use efficiency and water production efficiency. A method based on complex fuzzy matter-element method is proposed to evaluate the benefits of farmland irrigation water. Taking the irrigation water for rice crops in southern Jiangsu as a case, the entropy value and weight are calculated by entropy weight method, and the evaluation results of the benefits of rice water use over the years are calculated by Euclidean closeness degree. This conclusion can provide decision support for rational allocation of farmland irrigation water, efficient utilization of water resources and comprehensive reform of agricultural water price.