Developing modern circular agriculture is helpful to promote the high-quality transformation of agricultural production mode in China, which is one of the important ways to realize agricultural sustainable development. However, when the agricultural production operators applied the circular agriculture model to the particular system construction, there often occurred problems such as unknown input and output, low circulation efficiency and lack of overall evaluation, which led to obstacles for the efficient operation and material-energy utilization of modern circular agriculture systems. In this study, the modern "straw-sheep-cropland" agro-pastoral circular system of Donglin Village in Jiangsu Province was selected as the research object, due to its typical representativeness in the research of modern circular agriculture. The whole system consisted of 4 subsystems as cereal cropping, feed producing, sheep raising and manure composting. On the four-dimensional spatial-temporal scale, the system boundary was defined by drawing the emergy flow diagram. Hence the input and output items of the case system were comprehensively sorted out covering the entire year operation. On this basis, aiming to simulate the gradual progress of subsystems combination, system circulation and optimal regulation, 4 development stages were set as the separate cropping and raising, the connective cropping and raising (by producing or composting), the present circular, and the optimized circular. Then, the emergy input-output and structure of subsystems along with the whole system in different stages were quantitatively evaluated and comparatively studied by emergy analysis. The results showed that the gross emergy input of the feed production subsystem was the largest among 4 subsystems at the present circular stage, as its designed production scale needed a large amount of corresponding resources such as purchased straw to be invested. Meanwhile, unit emergy value of mutton from the sheep raising subsystem was the highest due to its role of consumer in the ecosystem. During the gradual process of constructing the industrial chain of the agro-pastoral system step by step until circulating production, the gross emergy input of the whole system increased obviously with the augmentation of subsystems, so did the internal circulation rate of material-energy with the development of the system construction. Subsequently, targeted at intra-system absorption of waste, the present circular system was optimally regulated by strengthening the coupling level between subsystems. At the optimized circular stage, system required lower gross emergy input and consumed less external economic resource, which demonstrated a better low emergy consumption characteristic. And the recycling of planting and breeding waste as well as self-sustaining production within the system resulted in significant enhancement of local resources utilization degree in each subsystem. In the meantime, unit emergy values of effective products and by-products produced at this stage all increased, which was mainly on account of the higher emergy level for driving the circulating operation of the system by adding extra material, energy and information. This study quantitatively evaluated emergy input and resource utilization during the construction and optimization process of modern agro-pastoral circular system based on empirical study, which could not only provide parameter support for efficient circulating operation of the system, but also supply accurate guidance for scientific system construction and replicated model promotion of circular agriculture.