Digital infrastructure construction is an important driver for the transformation and upgrading of traditional productive forces, so it is of great significance to clarify how it enables the improvement of new quality productive forces. Based on panel data from 277 prefecture-level cities in China from 2011 to 2022, this paper establishes a scientifically validated metric for new quality productive forces. Leveraging the phased implementation of the “Broadband China” pilot policies initiated in 2014 as a “quasi-natural experiment”, it employs a multi-period difference-in-differences (DID) design to estimate the benchmark impact, heterogeneity impact, mechanism, and spatial spillover effects of digital infrastructure construction on new quality productive forces. The results show that digital infrastructure construction has significantly improved the level of new quality productive forces, and this conclusion is verified in the treatment of endogeneity problems and various robustness tests. Compared with central and western cities and smaller cities, the positive effect of digital infrastructure construction on new quality productive cities is more significant in eastern cities and larger cities. Economic agglomeration and industrial upgrading are important mechanisms for the impact of digital infrastructure construction on new quality productive forces, that is, digital infrastructure construction can promote economic agglomeration and industrial upgrading, and then enhance new quality productive forces. Digital infrastructure construction exhibits pronounced spatial spillover effects, with initiatives in pilot cities enhancing new quality productive forces in geographically adjacent or economically similar neighboring areas.