The composite quasi-natural experiment empowered by the "dual-carbon linkage" was reconstructed based on carbon emission trading and low-carbon city construction. Using panel data from 280 prefecture-level cities in China from 2011 to 2022, a double machine learning model was employed to examine the impact of the "dual-carbon linkage" on urban green transformation. It was found that the "dual-carbon linkage" had a significant positive effect on both urban economic transformation performance and green innovation performance, with coefficients of 0.027 and 0.083, respectively, significant at the 1% statistical level. This indicated that the "dual-carbon linkage" contributed to comprehensively accelerating urban green transformation and demonstrated a more pronounced empowering effect compared to single institutional innovations. Mechanism analysis based on the "government-market-society" three-dimensional framework revealed that the "dual-carbon linkage" indirectly facilitated urban green transformation through government environmental improvement effects by reducing environmental pollution and enhancing environmental enforcement (with coefficients of -0.727 and 0.721, respectively), market resource allocation effects by promoting industrial upgrading and accelerating economic agglomeration (with coefficients of 0.134 and 0.332, respectively), and social participation and governance effects by raising public environmental awareness and optimizing public service environments (with coefficients of 0.487 and 0.052, respectively). Heterogeneity analysis showed that the incentivizing effects of the "dual-carbon linkage" on green transformation were more pronounced in cities with lower bureaucratic redundancy (coefficients of 0.110 and 0.147, respectively), greater financial technology support (coefficients of 0.039 and 0.067, respectively), higher market integration (coefficients of 0.057 and 0.102, respectively), better sci-tech business environments (coefficients of 0.121 and 0.138, respectively), as well as in cities located in the Yangtze River Economic Belt (coefficients of 0.041 and 0.069, respectively) and eastern regions (coefficients of 0.063 and 0.046, respectively). This study provided empirical evidence and decision-making insights for local governments to coordinate and unleash the synergistic force of "dual carbon" institutional innovations and accelerate the comprehensive green transformation of the economy and society during the "15th Five-Year Plan" period.