Efficient tax administration is of great significance for optimizing resource allocation and guiding corporate investment behavior. It holds strategic significance in breaking down local protectionism and market fragmentation, thereby advancing the development of a unified national market. Based on over 5.9 million cross-regional investment records from 2010 to 2022, this study employs a multi-period difference-in-differences model to empirically examine the impact and mechanisms of tax big data platforms on firms’ cross-regional investment. The findings indicate that the establishment of such platforms significantly promotes cross-regional investment, a conclusion that remains robust after a series of tests including parallel trend analysis. Mechanism analysis reveals that tax big data platforms facilitate cross-regional investment through the combined effects of fairness, burden reduction, and improved governance. Heterogeneity analysis further shows that at the regional level, the effect is more pronounced in areas with lower fiscal pressure, inter-provincial investment flows, and lower density of financial outlets. At the firm level, non-state-owned enterprises and those with higher tax compliance exhibit stronger responsiveness. These insights offer practical guidance for leveraging tax big data to enhance the role of tax administration in supporting the construction of a unified national market.