The China-Europe Railway Express (CRE) and Pilot Free Trade Zones (FTZs) are pivotal engines of China’s infrastructure connectivity and institutional openness, as well as robust pillars for enterprises to achieve integrated development of domestic and international trade. Taking the CRE and FTZs as a quasi-natural experiment, and 1615 A-share listed enterprises from 2012 to 2022 as the research sample, this paper employs a multi-period DID model to empirically test the impact and mechanism of the superposition of “hard connectivity” and “soft connectivity” on the integrated development of domestic and international trade. The findings are as follows: First, both the CRE and FTZs can significantly boost the integrated development of domestic and international trade, with a notable superposition effect between them. Second, under the combined effect of the CRE and FTZs, state-owned enterprises exert a demonstration effect in the integrated development of domestic and international trade; the promotion effect is most pronounced for labor-intensive enterprises; and from the perspective of enterprise life cycle, their combined effect exhibits a U-shaped characteristic. Third, from the perspective of the market integration mechanism, the CRE and FTZs can enhance domestic market integration and strengthen international market linkage, thereby facilitating the integrated development of domestic and international trade; while from the perspective of the market expansion mechanism, they can reduce cost stickiness and alleviate financing constraints, thus boosting the integrated development of domestic and international trade. Fourth, the digital transformation of enterprises acts as a catalyst for the CRE and FTZs to jointly promote the integrated development of domestic and international trade, with the most prominent catalytic effect observed in the western regions of China. In conclusion, advancing the integration of “hard connectivity” and “soft connectivity” through the collaborative development of the CRE and FTZs to support the integrated development of domestic and international trade is of great significance for fostering a new development paradigm.