Abstract: Studying the spatial-temporal driving mechanism of urban land expansion is of great significance to the scientific sustainable development and national spatial planning of urban agglomerations. Based on the General Additive Model (GAM), this paper selects socio-economic factors, natural factors and neighborhood factors to analyze the spatial-temporal differentiation pattern of the urban land expansion driving forces in urban agglomeration in the middle reaches of the Yangtze River from 1990 to 2015. We then explore the core driving force of each sub-urban agglomeration and identify its function. The results show that: (1) In the past 25 years, the spatial and temporal differences of driving factors of urban agglomeration in the middle reaches of the Yangtze river were significant. The core cities Wuhan, Changsha and Nanchang had obvious polarization effects and gradually appeared as the trickle-down effect. However, the urban agglomeration still had not broken the administrative division barriers. (2) The core driving factors of the three sub-urban agglomerations manifested significantly different. Among them, the population and GDP per area had the greatest impact on the urban land expansion in each sub-urban agglomeration. The spatial spillover effect of the Wuhan metropolitan area and the urban agglomeration around Poyang Lake was higher than that of the Changsha-Zhuzhou-Xiangtan urban agglomeration, and the expansion of urban land use changed from a single-core development mode to dual-core development mode. (3) There was a significant non-linear relationship between the core driving factors and the urban land expansion of urban agglomerations, and the characteristics of regionalism and stage were presented. (4) The GAM model can explain the nonlinear relationship between various factors in the process of urban land expansion, which had higher goodness of fitting and verification accuracy than Logistic and Nonliner-Logistic models, indicating its advantages in exploring non-linear driving forces.