Carbon dioxide is the core variable affecting the harmonious coexistence between human beings and nature. Urban agglomerations (UAs) are the main regions in China that emit carbon dioxide, and they are also regions with stronger economic and technological strength. Carbon emission reduction (CER) in UAs is an inevitable choice for modernization of harmonious coexistence between human beings and nature. In the face of the arduous task of CER, key UAs and key industries need to be focused on. Based on whether the UAs are carbon peaked or not, as well as their carbon emissions, CER power and capacity, six UAs, namely the Yangtze River Delta, Shandong Peninsula, the middle reaches of the Yangtze River, Hohhot-Baotou-Erdos-Yulin, the coastal areas of Guangdong, Fujian and Zhejiang, and the Pearl River Delta, are the key UAs in terms of CER before 2030. The carbon emissions of each UA have significant industrial concentration. Cluster analysis reveals that 10 industries, such as electricity, steam and hot water industry, constitute the main body of carbon emissions and are the key industries for CER. To promote CER in UAs, it is necessary to improve the market mechanism and the role of the government, combine the characteristics of high-emission UAs and high-emission industries, accelerate innovative emission reduction, and realize the “win–win” of green ecology and industrial upgrading.