Air traffic complexity is the essential attribute of air traffic situation, and it is therefore one of the drivers for air traffic controller"s workload. Scientific evaluation of air traffic complexity is the key to realize air traffic refined management and is a research hotspot of air traffic management. This study introduces the Chinese and global research teams on air traffic complexity, summarizes the differences and associations of air traffic complexity with workload and safety level, and elaborates the concrete clues in the existing research. Then the future research trends and directions of air traffic complexity are prospected on basis of characterization of new-generation air traffic management system.