With the rapid urbanization, the urban population continues to increase, and the land use changes increasingly, which may induce the deterioration of ecosystems and cause threat to urban sustainable development. As an effective tool of systematic analysis, the concept of urban ecosystem health can be introduced to comprehensively evaluate the development status of cities and provide a systematic diagnosis for urban ecosystems. It is found that existing health studies on urban ecosystems mostly focus on the scale of cities and urban agglomerations, but few studies have concretely analyzed differences within cities, which is right important for urban ecological management. In order to further explore the characteristics of health distribution within cities, we paid attention to the ecosystem health at county scale in this paper. After analyzing the key characteristics of ecosystem health, we first proposed a new four-dimensional evaluation framework of county ecosystem health including "development level-service function-resistance of disturbance-maintenance". And then we established a multi-layer evaluation index system of county ecosystem health composed of 20 indicators. The evaluation model of weighted sum was also constructed. In addition, the key contribution factors of county ecosystem health were identified based on the analysis of change of county ecosystem health and spatial differentiation features. It is indicated that the state of ecosystem health showed a common tendency of deterioration for the 32 counties of Dongguan from 1995 to 2015, while the degradation of the southwest region (e.g., Houjie and Shatian) was more serious than those of the southeast region (e.g., Huangjiang and Qingxi). Among the four dimensions of "development level-service function-resistance of disturbance-maintenance", we found that maintenance has been exerting a great impact on the county ecosystem health, leading to changes of ecosystem health. It is further found that there were two main limiting factors of country ecosystem health, i.e., the scarcity of land and its low usage efficiency. To improve the urban ecosystem health level of Dongguan, it is suggested to improve the usage efficiency of land and other resources, and further adjust the development model and industrial structure. For example, we can release the land stock market by three-old transformation; promote ecological protection and especially ensure the health of water, soil and forest; make use of the location advantages of Dongguan to optimize the industrial structure of various counties. It is concluded that the study on county ecosystem health can help decision-makers better understand the complicated internal characteristics in urban ecosystems, which is useful for balancing regional development and maintaining the health of urban ecosystem.