Based on the idea of "learning from nature", Zhang Guoqing put forward three theories of complex system, namely, ecology theory, health theory and harmony theory. Zhang Guoqing believes that the "inaction" realm of "learning from nature" is "being born for heaven and being born for the world", and implementing "natural" governance, that is, "learning from heaven and building good laws; Governing by doing nothing, loving everything by following the good, or "ruling everything by the good way", this artistic expression of "learning from nature" is the aspiration of the people and the personalized expression of "learning from nature". Ecological theory is the basic theory and method to explore the world, study and guide the development of human society by using ecological thought. The origin of ecological theory is "learning from nature". The theories and methods of ecology not only come from biology, mathematics, physics, chemistry and other disciplines, but also are applied to various disciplines including the above-mentioned disciplines after fusion and refining. This cycle makes our methodology progress continuously. Health theory is a theory about system health and its management. System health means that the system has a harmonious and stable structure, can continuously play a perfect function, and is safe and reliable. System health management (SHM) refers to the scientific and reasonable development, utilization and protection of the system according to the health status of the system and on the basis of observing the operating rules of the system, so that the structure and functions of the system can operate efficiently, harmoniously and continuously. Harmony theory is a theory about system harmony. Harmony means that the organizational structure of the system is in the best state and can fully and continuously function. Harmony, for a generalized system, is the best state of the system, the ideal goal of the system and the evolution process of the system towards the ideal goal.