Start-ups are relatively disadvantaged in terms of resource access and allocation. In particular, the financing constraint brought by traditional financial exclusion is not conducive to the sustainable growth of start-ups. Inclusive and accurate digital finance can provide more financial services at a lower cost to start-ups, thus facilitating their growth. However, the existing literature on the impact of the development of digital finance on the growth of start-ups is scarce, and there is a lack of empirical evidence.This paper believes that the development of digital finance can create a favorable investment and financing environment for start-ups, improve their ability to acquire and allocate financial resources, and thus promote the growth of start-ups. The analysis of the sample of SME and GEM start-ups from 2012 to 2020 shows that: (1) the increase in the level of digital financial development in cities has a significant positive effect on the growth of start-ups. The partial mediation effect of financing constraints and the chain mediation effect of “financing constraints - R&D investment” are significant. It shows that the development of digital finance can promote the growth of start-ups by easing financing constraints, and there is a transmission path of “digital finance development → alleviating financing constraints of start-ups → increasing R&D input of start-ups → promoting the growth of start-ups”; (2) the positive impact of the development level of urban digital finance on the growth of start-ups is significant in the eastern region and areas with higher digital level, but not significant in the central and western regions and areas with lower digital level, indicating that the increase in regional market size, degree of marketization, level of openness to the outside world and digitalization levels is conducive to the development of digital financial functions and roles; (3) the increase in the level of digital finance development in cities has a significant positive impact on the growth of non-state start-ups with stronger financing constraints and high-tech start-ups that rely more on technological innovation, while it has no significant impact on the growth of state-owned start-ups and non-high-tech start-ups. This indicates that financing constraint is one of the main obstacles to the growth of start-ups, while technological innovation is one of the main paths for start-ups to enhance their growth capability. Compared with the existing literature, this paper discusses the impact of the development of digital finance on the growth of start-ups, provides empirical evidence on digital finance development promoting the growth of start-ups and its chain mechanism and heterogeneity, and expands and deepens the research on digital finance supporting the development of the real economy as well as the research on the influencing factors of enterprise growth. This paper shows that there is a “long-tail effect” in digital finance development, which can help solve the problem of “first-mile” blockage in the early financing of enterprises. Therefore, all regions should adhere to deepening reform and opening up, actively promote the development of the digital economy, continuously improve the level of marketization and digitalization, and give full play to the positive effects of digital finance; financial institutions should pay attention to the financial needs of various enterprises, optimize financial services and innovate financial products in a targeted manner, and better serve the development of the real economy; start-ups should make full use of the convenience and opportunities brought by the development of digital finance, strengthen technological innovation, and thus achieve high-quality sustainable growth.