Since 2000, China has been upgrading its infrastructure, exemplified by the expansion of the high-speed railways (HSR), while simultaneously moving up the global value chains, evidenced by the rising domestic value-added ratio (DVAR) in exports. To analyze the impacts of the HSR on China's DVAR, this paper develops a new methodology to estimate firm-level DVAR using only customs transaction data. This paper also proposes a novel way to capture firmlevel exporter-supplier input-output linkages by matching the custom product codes of importers and exporters. The results show that HSR can explain 14 percent of the increase in firm DVAR for 2007-2016, as it increases the share of domestic suppliers connected to exporters. A two-sector model with a quantitative analysis shows that HSR improves the probability of exporters connecting with low-communication cost domestic suppliers, leading to lower domestic material prices and higher varieties of accessible domestic materials, hence pushing up firm DVAR.