Against the backdrop of sustained global growth in cross-border e-commerce, the Western region, while demonstrating impressive growth rates, faces significant inter-provincial gradient imbalances. Based on niche theory, this paper constructs an evaluation system encompassing six dimensions: foreign trade, economic development, e-commerce logistics, talent environment, technological innovation, and openness to the outside world. It employs entropy weight-TOPSIS measurement and K-means++ clustering to assess the cross-border e-commerce competitiveness of 12 Western provinces from 2021 to 2025. The results reveal that Sichuan, Chongqing, and Shaanxi form a “Golden Triangle,” Guangxi and Yunnan constitute a growth belt, Xinjiang, Guizhou, Ningxia, and Gansu are identified as potential areas, while Qinghai, Tibet, and Inner Mongolia remain at a disadvantage. Sichuan and Chongqing share a niche overlap of 0.78, indicating the fiercest competition, whereas their overlap with Tibet is only 0.21, suggesting dislocation and complementarity. The Western provinces exhibit a spatial pattern of “dual-core leadership with gradient decline.” The study further proposes a four-dimensional differentiated policy pathway encompassing “channels, industries, institutions, and talent,” providing a decision-making basis for formulating cross-border e-commerce development plans in Western provinces.