The sustainable development of agriculture in the agro-pastoral ecotone has always been an important topic of regional agricultural research in China. In this paper, Yanchi County in Ningxia was taken as a typical case area of the agro-pastoral transitional zone in north China; a ecosystem services and dis-services of the crop ecosystem in Yanchi County from 1991 to 2016 were evaluated by using the framework of emergy analysis of an agricultural ecosystem, to try to reveal the effects of a grazing-prohibition policy on sustainable development of crops. The results showed that:(1) From 1991 to 2016, the emergy input, ecosystem services and dis-services of the cropping ecosystem in Yanchi County increased. After the implementation of the grazing-prohibition policy, the growth rate increased. The increase of emergy input in cropping was mainly due to the increase of purchased emergy input. (2) According to the Pearson correlation analysis, there was a significant positive correlation between the ecosystem services and anti-services at the 0.01 level (bilateral), and changes in agricultural ecosystem services and dis-services had the same trend. (3) The implementation of the grazing-prohibition policy promoted an improvement in the ecological environment, the increase in the purchased emergy input of the social economic system, and the increase in irrigated area, which can enhance the ability of the semi-arid northern agro-pastoral ecotone to withstand climate fluctuations. (4) The sustainability of crops in Yanchi County was higher than the national average and that of typical agricultural areas, and its sustainability showed a steady downward trend before the implementation of the grazing-prohibition policy, whereas an upward trend was observed after the implementation of this policy. The implementation of the grazing-prohibition policy has led to preliminary ecological and economic benefits for agricultural production.