Abstract: With the method of accessibility, this study defined a county-scale in urban agglomeration in the middle reaches of Yangtze River. Based on the entropy method, this study estimated a comprehensive evaluation index system for county urbanization level in three subsystems: population, economy and society in 2001, 2006 and 2011. Spatial autocorrelation was used to analyze the spatial evolution of the comprehensive level of urbanization. Regression analysis was used to analyze the driving factors for each type of county. The following conclusions are made: (1) there were three nodes i.e. Wuhan, Changsha and Nanchang through calculation of the city's comprehensive strength, which defines the county-scale in urban agglomeration in the middle reaches of Yangtze River based on "3 hours city circle" scope with grid-network integration method with ArcGis9.3. It has 163 counties with the method of accessibility. (2) The comprehensive level of county urbanization continually improving, but the difference of the evolving speed is obvious in urban agglomeration in the middle reaches of Yangtze river; for the subsystems, economic evolution is the main feature of the rapid evolution of urbanization, followed by the social urbanization and the population urbanization process is the least advanced. It showed that county will enter or has already entered into the second phase of urbanization (the connotation of urbanization stage) in urban agglomeration in this area. (3) The comprehensive urbanization level and changing rate were closely related to city level and scale, the transportation and transformation of industrial structure in 2001, 2006 and 2011; Each subsystem has its own characteristics of spatial evolving pattern, but the changing rate has obvious similarity among subsystems. (4) According to the correlation of comprehensive urbanization level, the county can be divided into four types, i.e. coupled together, central lowland, antagonism together, and core-edge type; the evolution of spatial pattern is obvious in each type of county. (5) Endogenous force in "coupled together" county and "antagonism together" county is an acceleration role in the process of urbanization, investment force effect on depression in "central lowland" county, the main driving factor for "core-edge" county is administrative force and endogenous force. Endogenous force had a significant effect in four types of counties. It showed that the regional urbanization largely depended on its own economic development or production elements. Based on the above results, this study put forward some corresponding measures to promote the development of urbanization and guide the healthy development of regional urbanization based on various types of county driving factor. It is of far-reaching significance that urban agglomeration in the middle reaches of Yangtze River is striving to become "the fourth pole" of Chinese economy.

Key words: accessibility, ESDA, urbanization, driving factors, urban agglomeration in the middle reaches of Yangtze River