Objectives: The urban agglomeration has evolved into the main form of China's urbanization in the next stage, which can greatly affect the urban spatial pattern in China. At present, although the research on the expansion of urban agglomerations is deepening and becoming mature, there still exists some problems, e.g., insufficient quantitative measurement on the expansion form of urban agglomerations, neglection of the analysis on the spatial differentiation of expansion modes and lack of the spatial interaction factors in driving forces. In view of this, taking Chang-Zhu-Tan Urban Agglomeration (CZT-UA), which is the earliest integration in central China and is of strategic value to the rise of the central region, as the case study, a comprehensive study has been conducted on the spatial expansion of CZT-UA as well as the interaction between different cities. Methods: 30m impervious surface data is firstly used to extract time series of CZT-UA's built-up area from 2003 to 2018 owing to its time-sequence stability and accessibility. Then a collection of measurement methods, e.g., fractal dimension, expansion intensity index, Moran's I and Getis-ord Gi* are applied to quantitatively reveal the spatial structure, expansion process and spatialtemporal patterns of CZT-UA's built-up area. Finally, principal component analysis and geographically weighted regression are integrated to explore the driving forces for the built-up area's expansion in CZT-UA. In this regression model, two special factors are established to represent the interaction inside the urban agglomeration besides the traditional population, social economy and transportation accessibility factors. Results: The results show that:(1) The CZT-UA's built-up area shows obvious axial distribution, which generally follows Xiangjiang River and the transportation network consisting of five vertical and five horizontal trunk lines. (2) From 2003 to 2018, the built-up area and expansion speed of Changsha, Zhuzhou, and Xiangtan showed an overall upward trend. Compared with Changsha, Zhuzhou and Xiangtan expand more slowly and the gap is gradually widened, and thus Changsha's dominant position in the urban agglomeration continues to strengthen. (3) The expansion pattern analysis shows that the overall spatial differences in the expansion and distribution of built-up area in CZT-UA have gradually narrowed. In addition, the hotspot regions of urban expansion formed a kernel in central CZT-UA, and exerted a driving force to the peripheral areas. (4) The GWR regression model demonstrates that three principal factors are involved in the expansion of CZT-UA's built-up area, including socio-economic external connections, location and traffic accessibility, and administrative power. The model output indicates that the flow of residents and economy between cities, clear policy guidance from government, convenient transportation network as well as the radiation effect of the central city can jointly attribute to the expansion of built-up areas. However, each factor shows different importance, among which the administrative power has the most positive impact, and the northern parts in CZT-UA are more sensitive to it; the driving force of location and traffic accessibility decreases from the north to the south, while socioeconomic external connections show a much more important effect on the expansion of the southern CZT-UA. Conclusions: Based on the thorough analysis, this paper suggests that relevant decision-making departments should further promote the integration and sustainable development of CZT-UA by strengthening the allocation of resources and financial support to Zhuzhou and Xiangtan through appropriate policy tilt and giving full play to the radiation function and trickle-down effect of Changsha to narrow the regional gap in the urban agglomeration. Given that expansion of the built-up area in CZT-UA has been significantly attracted by road network, it is feasible to improve the construction of comprehensive transportation infrastructure such as intercity expressway in remote areas to enhance the accessibility and interaction between regions.