Under the requirements of high-quality development and regional coordination, the construction of the “Belt and Road” Initiative in southwest China not only needs to consider the radiation capacity of the policy center city, but also needs to focus on its impact on the surrounding areas. Based on the panel data of 13 central cities of the “Belt and Road” in Southwest China from 2014 to 2021, using the economic radiation model under Fourier’s law, the economic radiation capacity of the central city was measured based on the growth pole theory, center-periphery theory and industrial gradient theory. Based on this, considering the moderating effect of industrial isomorphism and the stage characteristics of the radiation effect, the high-dimensional fixed-effect model and threshold regression model were used to empirically analyze the economic radiation effect of the central city, and further verify it through the “U” test. The results show that the “siphon” and “feedback” effects of the economic radiation of the central cities of the “Belt and Road” in Southwest China exist at the same time and show phased characteristics with different economic radiation intensities, but the long-term performance is “feedback” effect, which is strengthened by industrial isomorphism, indicating that the implementation of the “Belt and Road” policy has a significant role in promoting the development of Southwest China.