Abstract: Medium-sized cities possess limited resources, such as land, capital, and talent. Accordingly, improving the primacy of the main urban area can effectively concentrate resources and give full play to economies of scale, ultimately enhancing the overall competitiveness of the cities. This study investigated Jiaxing City in Zhejiang Province as an example. Based on nighttime light (NTL) remote sensing data from the National Polar-orbiting Partnership-Visible Infrared Imaging Radiometer Suite (NPP-VIIRS), this study conducted a systematic analysis of the primacy evolution of the main urban area in dispersed medium-sized cities using a series of technical methods, such as spatial statistical analysis, standard deviational ellipse (SDE), and rank-size rule. The results indicate that from 2013 to 2022, the NTL in Jiaxing City experienced a temporally significant upward trend and a spatial distribution pattern characterized by overall growth with sharp increases in key areas. From 2013 to 2022, the urban expansion of Jiaxing was primarily driven by the joint influence of Shanghai and Hangzhou. In this process, the main urban area proactively expanded, connecting the Pinghu and Tongxiang cities under its jurisdiction. In contrast, the other three cities and two counties under its jurisdiction showed limited driving force in integrating into its main urban area. Under the current administrative division, Jiaxing exhibits a weak primacy of the main urban area, coupled with an unsustainable urban level. Therefore, this study proposed incorporating Tongxiang City into the main urban area through urban integration or by transforming the city into a district of Jiaxing. This measure will enable the new main urban area to maintain a high urban primacy in the long run, facilitating urban population concentration and further contributing to the central rise of Jiaxing City. The results of this study can serve as a valuable reference for improving the primacy of the main urban area in medium-sized cities and accelerating the high-quality development of the county economy.