The synergistic development of digital transformation and carbon neutrality is a key path to global sustainable development. This study utilized the Broadband China pilot policy and a multi-period Difference-in-Differences (DID) design to comprehensively evaluate the impact of digital infrastructure on carbon neutrality using panel data from 291 prefecture-level cities in China from 2008 to 2021. The results indicate that regional digital infrastructure contributed to carbon neutralization innovation, carbon-intensive industry innovation, and carbon-emission efficiency in the pilot areas. This held true after analyzing the results with propensity score matching, coarsened exact matching, robust estimators, and instrumental variable regression. Specifically, the results of the mediation effect model indicate that digital infrastructure promotes the process of carbon neutrality through several mechanisms, including promoting the rationalization of industrial structure, promoting digital economic development, and inducing innovation in digital technology. In addition, cities with better economic development, lower resource dependence, and weaker environmental constraints have a greater carbon neutrality dividend effect from regional digital infrastructure. These findings indicate a need to optimize policies to unleash the dividends of digital infrastructure in carbon neutrality, which can drive technological, structural, and efficiency changes through digital empowerment. In addition, cities with heterogeneous endowments need to develop targeted measures.