As digital economy becomes a trend to achieve high-quality economic development and green low-carbon transformation, digital infrastructure also rises to the level of national strategy. In this context, in-depth exploration of the intrinsic impact mechanism of digital infrastructure on carbon emissions, and systematic analysis of whether the Broadband China policy has dynamic sustainability and spatial spillover effects from theoretical and empirical perspectives are of great practical significance for the further implementation of the national strategies of Network Power and Digital China, and for the advancement of the construction of digital infrastructure and the exploration of new urban development models and governance models for low-carbon development. This paper puts digital infrastructure and carbon emission into the same framework, explores the intrinsic influence mechanism and spatial-temporal dynamic effect of digital infrastructure on carbon emission from the theoretical level, and innovatively incorporates double-difference into the construction of spatial matrices. Based on the Broadband China strategy as a quasi-natural experiment, using the panel data of prefecture-level cities in China from 2006 to 2019, this paper utilizes the asymptotic double difference method, spatial model and mediation effect model to test the intrinsic mechanism and spatial-temporal dynamic effect. The results show that the Broadband China policy has a significant inhibitory effect on carbon emissions, and this conclusion still holds after the robustness tests of instrumental variables method, triple difference method, and PSM-DID. Impact mechanism analysis shows that digital infrastructure can both directly affect carbon emissions and promote carbon emission reduction through industrial structure upgrading and green technology innovation. Spatial-temporal dynamic effects analysis shows that the impact of the Broadband China policy on carbon emissions has a time lag, and it will be played only in the second year, but the marginal effect shows an increasing trend over time. Moreover, the Broadband China policy has a positive spatial spillover effect on carbon emissions in geographically and economically similar regions. Heterogeneity analysis reveals that the carbon emission reduction effect of the Broadband China policy is not significant in the western region, but more obvious in the eastern and central regions, and the effect in the central region is larger than eastern region. Therefore, the article suggests continuing to improve the digital infrastructure, strengthening the promotion of the pilot policy of Broadband China; implementing dynamic and differentiated strategies according to local conditions; accelerating the transformation and upgrading of the industrial structure, improving the level of green technological innovation, and smoothing the intermediate channels of digital infrastructure for carbon emission reduction.