Abstract: A reasonable spatial structure is an essential foundation for urban agglomerations to achieve regional economic synergistic development. The use of statistical data to analyze changes in urban agglomerations has problems such as weak presentability and difficulty of data acquisition. To address this problem, this study proposed a method for extracting the built-up areas by using the nighttime light remote sensing data and land use data. Considering the Middle Reaches of Yangtze River Urban Agglomerations (MRYRUA) as a case study, this study presented a two-step method for the spatial-temporal fractal analysis. First, the spatial and temporal evolution characteristics of spatial structure within the urban agglomerations were analyzed on the basis of the results of the built-up areas of the MRYRUA from 1990 to 2020. Furthermore, with the help of fractal theory and the combination of morphological, correlation, and aggregation dimensions, the morphological, correlation, and aggregation characteristics of the spatial structure of the MRYRUA were discussed. It was observed that within the period of 25 a, the MRYRUA exhibited marginal expansion from the center to the surroundings. In contrast, the urban built-up areas within the MRYRUA were distributed along the rivers in a more pronounced strip-like pattern. The results of the morphological dimension indicated that the development of the Wuhan Metropolitan was relatively well-managed, with a stable and orderly expansion of cities. The results of the correlation dimension demonstrated that the structure of the Chang-Zhu-Tan Urban Agglomeration exhibited a higher degree of similarity, suggesting a more effective synergy of development. The aggregation dimension indicated that the intensity of expansion of cities in the Urban Agglomeration around Poyang Lake was inconsistent, suggesting that the core driving role was not yet stable. The findings of this study facilitated the full exploitation of the advantageous geographical position of the MRYRUA, thereby facilitating the balanced and integrated development of neighboring cities across provincial boundaries.