In the era of the digital economy, promoting the technological innovation of enterprises through network infrastructure construction has become an important path for high-quality economic development. However, the existing literature on the impact of network infrastructure construction on enterprise innovation performance and its mechanism is insufficient, especially lacking relevant empirical evidence. This paper believes that the improvement of regional (city) network infrastructure can enhance the innovation performance of enterprises by enhancing the regional knowledge spillover effect, promoting the accumulation of human capital, reducing the cost of enterprises, and promoting the digital transformation of enterprises. In addition, the improvement effect of innovation performance will show diversified enterprise heterogeneity due to different knowledge stock, scale, financing constraints, and R&D background of senior executives. Based on the practice of demonstration city construction in the implementation of the “Broadband China” strategy, the micro-data of A-share listed companies from 2010 to 2018 were adopted, and enterprises in three batches of 117 “Broadband China” demonstration cities (city clusters) from 2014 to 2016 were selected as the experimental group. The results of the analysis using the multi-period double-difference model show that the construction of “Broadband China” demonstration cities has significantly and steadily increased the number of patent applications of enterprises, and the promotion effect on the number of invention patent applications is more significant than the number of non-invention patent applications. The level of regional knowledge spillovers (including local knowledge spillovers and non-local knowledge spillovers), the degree of talent agglomeration, the cost of enterprises and the level of digital transformation of enterprises have significant partial mediating effects on the improvement of innovation performance of enterprises promoted by network infrastructure construction, and the knowledge spillovers path has the strongest explanation. Relatively speaking, the construction of network infrastructure has a stronger effect on improving the innovation performance of enterprises with more knowledge stock, larger scale, lower financing constraints, and senior executives with R&D background. Further analysis shows that the construction of “broadband China” demonstration cities is helpful to promote the exploratory innovation behavior of enterprises and improve the quality of enterprise innovation. Compared with the existing literature, this paper mainly makes the following expansions and improvements. Firstly, this paper expands the research on the economic effect of network infrastructure construction from the perspective of enterprise innovation, and conducts empirical analysis using the quasi-natural experiment of the “Broadband China” demonstration city. Secondly, this paper deeply analyzes the mechanism of network infrastructure construction affecting enterprise innovation, especially the role of knowledge spillovers, which provides theoretical reference and method reference for related research. The research of this paper shows that the construction of network infrastructure helps to break the geographical shackles of knowledge spillovers and realize the synergistic effect of knowledge spillovers’ local effects and field effects on enterprise innovation, which is the fundamental reason for promoting enterprise innovation. Therefore, all regions should strengthen the construction of network infrastructure, especially the less developed regions should strive to narrow the “digital divide”, and effectively reduce the financing constraints of enterprises, in order to give full play to the innovation promotion effect of network infrastructure construction.