The research uses the development of 2019 novel coronavirus disease (COVID-19) in the human body as an example to explore the microstructures and dynamic processes of a concise complex system from the lens of the five-phase system.Based on the structural balance theory and system dynamics,the research finds that transitive triads and cyclic triads in the fivephase system are both imbalanced.The integration of these differentiated triads comprises of a balanced intermediate form in the shape of quadrangular cycles.These cycles serve as microstructures of the five-phase system,due to the inherent balancing feedback mechanism,and support the generation of resultants.The altation of quadrangular cycles drives the spiraling development of the whole system.By orderly and regular interweaving of signed directed links,the research provides a holistic,process-oriented demonstration for the development processes of COVID-19.It clarifies that the essence of the five-phase system is phase-transition processes with the quadrangular cycle as carrier and supporter,rather than the static aggregation of five elements.The research deepens the understanding of system nonlinearity by visualizing the circular causality and promotes the academic dialogue between the West process theory and the Chinese inherited notion of the fivephase system.