As China enters a new normal in economic development, green development and innovation-driven growth have emerged as critical pathways for China’s economic transformation and upgrading. The green credit policy and the pilot program for innovative cities serve as two key policy instruments that provide institutional support for enhancing corporate green innovation capabilities. This paper treats the issuance of the Guidelines for Green Credit and the launch of the innovative city pilot program as a quasi-natural experiment. Using a dataset of A-share listed firms in China from 2007 to 2023, it employs a difference-in-differences (DID) approach to systematically evaluate the synergistic effect of these two policies on the quality of corporate green innovation, and further investigate the underlying mechanisms and heterogeneous impacts of this synergy. The results show that the simultaneous implementation of the innovative city policy and the green credit policy significantly improves the quality of corporate green innovation. Firms exposed to both policies exhibit significantly higher green innovation quality compared to those unaffected by either or only one policy, and this conclusion still holds after a series of robustness tests and endogeneity treatments. Moreover, the combined policy effect is significantly stronger than the effect of either policy implemented in isolation. Notably, the order of policy implementation plays a critical role: firms first exposed to the innovative city policy and subsequently to the green credit policy experience a more pronounced improvement in green innovation quality. Mechanistically, the policy synergy of innovative city construction and green credit enhances the quality of corporate green innovation through a dual pathway: increasing firms’ innovation investment and accelerating their green transformation. Further heterogeneity analyses reveal that this synergistic effect is more pronounced among firms located in regions with higher administrative status, state-owned enterprises (SOEs), and labor- or capital-intensive firms. These findings underscore the critical moderating roles of policy implementation environments and firm characteristics in shaping policy effectiveness. This study makes the following contributions. Theoretically, it is the first to uncover the synergistic effect and its underlying mechanisms between green credit policy and innovative city construction policy, offering a novel perspective for the coordinated design and optimization of public policies and further deepening the understanding of the effectiveness of policy instruments aimed at fostering green innovation. Practically, the findings provide valuable policy insights for promoting the integrated development of digitalization and green development and for advancing the national agenda of building a great power of quality, and also offer guidance for firms seeking pathways to green transformation and upgrading in the context of China’s new development era. Furthermore, the study underscores the importance of inter-policy coordination, advocating for integrated, multi-layered policy planning to maximize synergistic impacts in achieving sustainable development goals.