Southwest China is a typical region characterized by rich natural resources but relatively low economic development, leading to unbalanced development. Using panel data from Southwest China spanning 2010 to 2022, this study employs the entropy weight method, coupling coordination degree model, and spatial econometric modeling to systematically examine the spatial spillover effects of high-speed rail network development on the coupled coordination between urbanization and ecological environment in the region. The key findings are as follows: The coupling and coordination degree of urbanization and ecological environment in Southwest China is predominantly at low and medium levels, with a continuous improvement trend, exhibiting typical characteristics of spatial agglomeration and disequilibrium; The betweenness centrality of the high-speed rail network has a significant positive impact on the coupling and coordination of urbanization and ecological environment; The high-speed rail network exerts a stronger promotional effect on coordination through indirect effects, demonstrating notable spatial spillover effects. In the long term, enhancing the high-speed rail network level is conducive to the harmonious development of the coupling and coordinated relationship between urbanization and ecological environment in Southwest China, and can narrow the development disparities among cities in the region. However, due to the more prominent indirect effect of the high-speed rail network on urban coordinated development, its impact exhibits a certain lag.