The concept of digital twin has developed rapidly in recent years and has achieved certain results in aerospace, machinery manufacturing and urban areas. This paper uses the ecological system theory to understand the digital twin city, and takes the digital twin city as a system which is composed of five scales: cells, tissues, organs, systems, and living bodies. However, with the development and practice of digital twin cities, it is still under exploration to realize real-time monitoring and data fusion between urban systems, and to support multi-dimensional intelligent analysis and decision-making in public services, economic industries, infrastructure and planning management. Since scale is the basis of cognitive complex systems, the subjects and rules of digital twin cities at different scales are also different, which can result in different design priorities for twinning practices at different scales. Therefore, this paper firstly understands the digital twin city from the system theory, studies the digital twin city as a whole. Second, it is divided into four scales: cell, organ, system, and living body; then we suggest to build a digital twin city scale system framework that conforms to the system theory. At last, three suggestions for building the system framework are put forward: to clarify the differences between subjects and rules at different scales; to provide technical solutions for design requirements at different scales; to ensure collaboration between multiple scales so as to achieve the goal of improving urban operation efficiency and residents' living comfort through digital twin city construction.