The broadband network is driving a new wave of informationization development, and many countries have prioritized broadband network development as a strategic area of action. It is also seen as an important initiative to gain a competitive edge in international economic, scientific, technological, and industrial competition in the new era. Digital infrastructure development, as a crucial driver of modernization, plays a pivotal role in fostering green and sustainable innovation. This study employs a quasi-natural experimental approach based on "Broadband China" pilot cities, using a differencein- differences (DID) model to explore the impact of digital infrastructure development on green and sustainable innovation in Chinese prefecture-level cities. Research results manifest that the construction of "Broadband China" significantly promotes green sustainable innovation. The capital deepening effect brought by large-scale investment and the popularization of high-speed broadband networks greatly improves information access and dissemination efficiency, optimizes resource allocation, reduces energy waste, and promotes the R&D and application of green technology. Heterogeneity analysis reveals that cities with a medium level of openness to the outside world benefit more significantly from the "Broadband China" policy. From the perspective of resource-dependent cities, the policy significantly promotes green and sustainable innovation in mature and non-resource-dependent cities. Mechanism analysis indicates that "Broadband China" fosters green and sustainable innovation through enhanced digitalization and improved energy efficiency, while unemployment rates negatively moderate the relationship between "Broadband China" and green sustainable innovation.