Abstract: In order to understand the characteristics and direction of current urban agglomeration development and the mechanism of construction land expansion in urban agglomerations, the Beijing-Tianjin-Hebei urban agglome-ration (BTH) and the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), the most representative urban agg-lomerations in China, are selected as the research objects. The spatio-temporal characteristics of construction land expansion in BTH and GBA and the differences between these two agglomerations are analyzed based on the construction land data from 1988 to 2017 with the methods of urban expansion rate, urban expansion intensity, urban expansion difference index and landscape expansion index. The factors in the construction land expansion in the two urban agglomerations are analyzed with the grey correlation model. The results show that BTH has entered the period of rapid urbanization and GBA has entered the mature period of urban expansion, with its internal development speed better than that of BTH. First, the area of construction land expansion in BTH is three times more than in GBA, but the expansion rate and expansion intensity of BTH are both lower than those of GBA. The process of construction land expansion in BTH is generally slower than in GBA. Second, there are significant spatio-temporal differences in the construction land expansion between two urban agglomerations: the expansion rate and intensity of construction land in GBA are significantly higher than in BTH in 1988-2007, but since 2008, they have become lower than in BTH and the gap has continued to enlarge. Third, there are significant regional differences in the process of construction land expansion between two urban agglomerations: the speed of construction land expansion in BTH varies from high to low along the line from the south from the northeast and the central. The speed of construction land expansion in the east of GBA is higher than that in the west. Fourth, new construction land patches in the two urban agglomerations are mainly edge-expansion and infilling-expansion, with the proportion of outlying-expansion gradually decreasing, shifting from diffusion to agglomeration. However, the agglomeration level of construction land expansion in GBA is higher than in BTH. Finally, BTH is mainly affected by resident population and GDP while GDP and gross investment in fixed assets are the main driving forces of construction land expansion in GBA.