Understanding the morphologies and mechanisms of construction land expansion (CLE) in border areas is beneficial for promoting industrial coordination and cross-border cooperation along the border, also addresses the research gap caused by the predominant focus on construction expansion in economically developed inland cities. This study conducted a comprehensive and systematic comparative exploration of the complex and multidimensional patterns of CLE in the large-scale, multinational sub-regional border areas of the peninsula. Using the Global Artificial Impervious Area (GAIA) dataset, the study innovatively developed a morphology determination model for identifying expansion patterns, integrating morphological characteristic indices. It investigated the intensity, temporal evolution, and morphological characteristics of CLE in the border areas of Mainland Southeast Asia from 1986 to 2021. Our findings revealed several key insights: (1) CLE in the border areas of Mainland Southeast Asia was rapid, with expansion hotspots concentrated in the border areas of China, Vietnam, and Thailand. (2) CLE presented the spatial distance heterogeneity effects and national differences in variation, indicating that CLE differed depending on the distance from the borderline and the country it is located. (3) Under the influence of natural geography, social process, economic cooperation, geopolitical security and policy system, CLE in the border areas presented three morphologies: expansion towards the borderline (ETB), expansion along the roads/rivers (EAR), and network-based expansion (NBE). Exploring the mechanism of spatial expansion of border construction and promoting rational planning of cross-border construction is a feasible path to build a pattern of complementary advantages of bilateral countries and coordinated regional economic integration.