As critical nodes in the globalized space that facilitate the agglomeration, diffusion, and allocation of production factors, trade hubs play a central role in connecting domestic and international markets, strengthening economic circulation, and reconstructing competitive advantages. Currently, domestic market segmentation results in differentiated adaptation of high-standard international trade and economic rules across different regions, which constrains China’s institutional opening-up. Trade hubs do not promote the domestic and international markets in isolation; rather, they also create spaces for the convergence and transformation of different regulatory systems between the two markets. Thus, promoting trade hubs that synergize “hard connectivity” and “soft connectivity”, and upgrading their function from mere channel connection to an integrated system for market resource allocation, is conducive to enhancing regional opening-up performance characterized by efficient interaction between domestic and international markets. This paper uses panel data from 219 prefecture-level and above cities from 2009 to 2022 to empirically examine the impact of constructing trade hubs that integrate “hard connectivity” and “soft connectivity” on regional opening-up performance and its underlying mechanisms. The results show that hub construction can significantly enhance regional opening-up performance, with the effect of composite trade hubs being stronger than that of single-channel connectivity. This conclusion remains valid after a series of robustness tests. Mechanism tests reveal that the construction of a unified national market is an important mechanism for trade hubs to connect with international high-standard economic and trade rules and strengthen regional openness. On the one hand, through two paths of relaxing market access and promoting market integration, trade hubs significantly promote the construction of a unified national market. On the other hand, the unification of the domestic market can strengthen the role of trade hub construction in promoting regional opening up, thereby promoting the linkage between domestic and international markets, fully leveraging the synergistic effect of international and domestic markets in cross-border resource allocation, resolving the inherent conflicts in system innovation and spatial reconstruction, and driving the expansion of regional foreign trade scale and the enhancement of openness intensity. Heterogeneity analysis shows that cities located outside major urban agglomerations in central and western regions, in economically peripheral areas, dominated by manufacturing, and in policy integration periods are more likely to benefit from the openness-promoting effects of trade hubs. Compared to existing studies, the marginal contributions of this paper are threefold: First, from an analytical perspective, it incorporates trade hubs, the unified national market, and regional opening-up into a single framework from the viewpoint of domestic and international market linkages, unveiling their complex intrinsic mechanisms. This provides a theoretical response to coordinating the strategies of “high-level opening-up” and “unified national market” under the new development paradigm to build a mutually reinforcing dual circulation pattern of domestic and international markets. Second, in terms of empirical strategy, targeting cities that concurrently host Pilot Free Trade Zones and China-Europe Railway Express nodes, it constructs a quasi-natural experiment of trade hub construction that combines “hard connectivity” and “soft connectivity”, offering methodological reference for extending the examination mechanism of the hub economy. Third, regarding policy implications, it explores the synergistic policy effects of high-level external opening-up and internal unified market construction, promotes the transformation of key urban nodes from channel-based network distribution nodes into multifunctional, integrated hub-type economic platforms, and provides economic rationale for removing institutional barriers in the transition from “border-to-hinterland” opening-up. This study depicts a circular cumulative process of high-level interaction between domestic and international markets. Specifically, the construction of trade hubs is far more than merely connecting domestic and international markets geographically. More critically, it operates through a mutually reinforcing mechanism of docking with international high-standard markets and empowering the domestic market scale. This mechanism transforms the vast domestic market into advantages of a more resilient and lower-cost international supply chain and a more attractive image as a global investment destination, ultimately leading to a substantive improvement in regional opening-up performance.