This paper conducts a comparative study on the development of global smart city clusters or belts, and analyzes their different developmental levels. Foremost, the corresponding evaluation methodology of the global smart city cluster is presented, and 9 indicators including traditional infrastructure, digital infrastructure, and institutional infrastructure are established. Moreover, it establishes the coastal city clusters or belts as analysis objectives including the Yangtze River Delta Region, the Guangdong-Hong Kong-Macao Greater Bay Area, the New York Bay Area, the Great Lakes Region, the three Bays of Japan, the Persian Gulf, the Gulf of Mexico, the North Sea and the Baltic Sea, as well as the inland clusters involving the Alps, the Carpathian Mountains and the Balkan Mountains, the Rocky Mountains and the interior of Asia. Then, these city clusters or belts are evaluated according to the indicators. After that, comparative studies of smart city clusters or belts construction are carried out to demonstrate their different development levels. The results conclude that the New York Bay, the Baltic Sea, and the North Sea lead the world in the construction of smart city clusters or urban belts. Surprisingly, the Guangdong-Hong Kong-Macao Greater Bay Area and the Yangtze River Delta Region smart city cluster are behind the world in terms of total scores and individual scores.