Abstract: Smart city construction is a critical pathway to promote green and low-carbon urban development, yet its mechanism for influencing the carbon total factor productivity requires further exploration.Based on the panel data of 108 cities in the Yangtze River Economic Belt (YREB) from 2007 to 2022, this study used the non-radial directional distance function (NDDF) to measure the carbon total factor productivity.While the smart city pilot policy is treated as a quasi-natural experiment, and through the multi-time point difference-in-differences model and the four-stage mediating effect model, this study systematically analyzed the direct effect, heterogeneity characteristics and green technology innovation transmission path of impact of smart city construction on the carbon total factor productivity.The results showed that: (1) The carbon total factor productivity in the YREB exhibited an overall upward trend with fluctuations, yet significant regional disparities existed, with a spatial gradient pattern of "downstream > midstream > upstream"; (2) Smart city construction significantly enhanced the carbon total factor productivity in the YREB, especially for midstream and downstream cities, large-sized cities and non-resource-based cities; (3) Green technology innovation served as the key intermediary channel for smart city construction to promote the carbon total factor productivity.Based on these results, it was suggested that the provinces and cities in the YREB establish regional collaborative mechanisms, implement differentiated smart carbon reduction strategies according to the endowments of cities, and strengthen the innovation and transformation of green technologies to promote the low-carbon transformation across the whole basin.