Abstract: Qingyang City is located in the hilly-gully region of the central Loess Plateau region，with arid climate and fragile environment and ecology. From the viewpoint of the ecology，with the related theories and methods of the ecological carrying capacity and the town system，the paper analyzed the evolution trend of the scale structure of the town system of the study area，adopted the ecological elasticity，the resource carrying capacity and the bearing pressure strength to make a comprehensive evaluation of  the ecological carrying capacity，and had a theoretical discussion on the relationship between the ecological carrying capacity and the town scale structure. On the basis，the scale structure of the town system in the study area had been adjusted and optimized，which provided the basis for the town development in the fragile ecological environment conditions. The results show as follows：（1） During 1985-2011，Qingyang City was in the initial formation of the town rating scale structure composed of the small cities，the country central towns，the key towns and the common towns，and the town population was mainly concentrated in the town centers and some key towns，then the central cities and some key towns developed faster，the cluster effect of the central city was stronger than diffusion effect，while the driving effects and the radiation effects of the whole region was weaker. For longer period of time in the future，the expansion of towns in the study area will mainly concentrated on the expansion of the scale of the central city and the key towns. （2）During 1985-2011，the ecological elasticity of Qingyang City were overall on the rise，which declines in smaller increments around 2000，then which gradually increased，and the change trend that reflected by the carrying capacity of resources was more or less in accordance with the ecological elasticity，while the resources pressure strength overall dropped，which was on the rise around 2000. Comparing to the different counties，the ecological capacity of Huan County is in the medium stable，medium carrying and low pressure level，and that of Xifeng District is in the medium stable，low carrying and high pressure level，Huachi County and Heshui County are in the medium stable，high carrying and low pressure level，Zhengning County，Ning County，Qingcheng County and Zhenyuan County are in the medium stable，medium carrying and medium pressure level. （3）There is an interacting dialectic relationship between scale structure of the urban system and ecological carrying capacity，this relationship is more obvious in the Loess Plateau region. Qingyang City downtown and parts of Zhengning，Qingcheng and Ningxian regions are taken as the major regional and key areas to promote urbanization to enlarge the scale of those regional towns which developed by mineral resources and other special advantage resources; other towns should also be steadily developed on the premise of ensuring ecological functions. Accordingly，the region will form a scientific and reasonable urban scale structure which the central city is a growth pole，focusing on the development of some cities and towns of Zhengning，Qingcheng and Ningxian，and with other area towns steadily developing.

Key words: ecological carrying capacity, hierarchical scale structure of town system, Loess Plateau region, Qingyang City