Sub-central city’s green innovation in regional integration: Can it be a bridgehead even if it is far away?

•Revealing the causal relationship between sub-central city and green innovation.•GTFP and GPG are used to measure green innovation.•The sub-center policy can promote green innovation (GTFP and GPG).•Results are of great significance to regional integration policy planning. The green innovation level of sub-center cities in regional integration must be addressed in promoting high-quality economic development. Therefore, governments and policymakers should pay attention to its positive effects. In the Yangtze River Delta integration context, this paper uses Hefei as an example sub-center city to assess its green innovation in two indicators, green total factor productivity (GTFP) and the number of green patent grants (GPG). The first step is to measure GTFP using the SBM-ML model. The second step is to evaluate the policy effects of sub-center city experiences on green innovation using Synthetic Control Methods. The empirical findings of this study indicate that sub-centers play an essential role in promoting green innovation even if they are located outside the core area of an urban agglomeration. Additionally, the placebo trial confirmed the reliability and significance of the results. This paper proposes some policy recommendations to realize global green innovation based on these findings.