On the basis of the analysis of the formation background of the concept “open complex giant system” in terms of the lectures, papers and letters published by Academician Qian Xuesen, this paper studies emphatically the abstract process of giant system → complex giant system → open complex giant system， introduces briefly the basic content of the theory of open complex giant system, and expounds that the formation of this theory has significance for development of science at present and in the future, especially for the investigation of system science, and noetic science—innovative thinking.