High-speed rail (HSR) connectivity can compress travel time and space distances and have a significant impact on trans-regional tourism flows. Using panel data on trans-regional flows of inbound tourism in 31 provinces of China from 2004 to 2019, this paper takes the HSR connectivity as a quasi-natural experiment and constructs a multi-period difference-in-differences (DID) model to empirically test the impact and the mechanism of the HSR connectivity on flows of inbound tourists. The results show that: The HSR connectivity has a significant positive effect on the trans-regional flow of inbound tourists; The connectivity of HSR has a greater impact on the trans-regional flow of inbound tourists in both western regions and regions with higher and lower tourism resource endowments. And the impact of HSR connectivity on trans-regional flows of inbound tourists is stronger in economically developed regions; the space-time distance in the trans-regional flows of inbound tourists can be compressed by the HSR connectivity. This paper provides policy recommendations for optimizing HSR routes to promote sustainable tourism development.