The development of urban agglomerations and smart energy systems in China are tightly connected. However, basic concepts of this interdisciplinary connection are still lacking. To fill this gap, this paper proposed an item of “Smart Energy Urban Agglomeration” (SEUA), and studied its driving mechanism, basic concepts in China, and indicator evaluation of typical urban agglomerations. Firstly, by a theorical analysis, the driving mechanism of SEUA was concluded as of “internal and external driven by two axes”, whereas national policies and cities’ spontaneity are external and internal driving forces, and two axes are urban planning and energy system integration. Secondly, using system approaches, its concept connotation, development barriers, and action suggestions were proposed based on China’s conditions. Thirdly, an indicator evaluation based on the driving mechanism was carried out to reveal the differences of four urban agglomerations, with relevant explanations. The results indicated that the Yangtze River Delta had the highest level of SEUA development because of its good performance in both axes. Meanwhile, the cities’ spontaneity had a stronger driving force than national policies. Therefore, it is urgent for China’s government to integrate the urban and energy planning among cities in urban agglomerations.