Abstract: With the methodology of reductionism, science and technology have achieved great success in the 20th century．The origin and evolution of the universe, fundamental elements and their basic interactions have all obtained deep scientific understanding．Since the beginning of the 21st century, emergent properties of various complex systems in the macroscopic world pose an important academic challenge in almost all disciplines．The systems science has become an important field of scientific exploration．Development of complex systems science shows that there are universal and simple laws behind complex phenomena．Exploring universal relationship between structure, environment and function of various systems, investigating general laws of evolution and regulation, are important tasks in complex systems science. In this paper, the latest research results in basic theory, living systems, climate change, social economy, intelligent technology, are introduced．These results show that exploring complexity under the methodology of systems science not only deepens the scientific understanding of specific systems, but also helps to reveal universal laws behind complex phenomena in different fields．