The construction of green and low-carbon circular (GLC) development economic system is conducive to the promotion of “carbon peaking and carbon neutral.” The level of GLC development in the Yangtze River Delta (YRD) region is related to the realization of the ambitious goal of “carbon peaking and carbon neutrality” in the region. This paper use principal component analysis (PCA) to process GLC development level of 41 cities in the YRD from 2008 to 2020. Then, we constructed panel Tobit model and threshold model from the perspective of industrial co-agglomeration and Internet utilization and empirically tested the influence of the two key variables on GLC development of the YRD. We found that (1) the YRD’s level of GLC development showed a dynamic evolution trend of “fluctuation, convergence, and rise.” The four provincial-level administrative regions of the YRD are in the order of GLC development level: Shanghai, Zhejiang, Jiangsu, and Anhui. (2) There is an inverted “U” Kuznets curve (KC) between industrial co-agglomeration and the development of GLC of the YRD. In the left segment of KC, the industrial co-agglomeration promotes GLC development of the YRD. In the right segment of KC, the industrial co-agglomeration inhibits GLC development of the YRD. Internet utilization enhances GLC development of the YRD. And the interaction of industrial co-agglomeration and Internet utilization cannot significantly enhance GLC development. (3) Double-threshold effect of opening-up is manifested as follows: industrial co-agglomeration on GLC development of the YRD goes through an insignificant-inhibited-improved evolutionary trajectory. Single-threshold effect of government intervention is manifested as follows: the impact of Internet utilization on GLC development of the YRD shifts from insignificant role to significant enhancement. In addition, there is an inverted-N type KC effect between industrialization and GLC development. Based on the above findings, we proposed suggestions in terms of industrial co-agglomeration, Internet-like digital technology application, anti-monopoly, and rational industrialization.
