The scientific meanings of safety are analyzed. The concept of “zaishi” is introduced and the fact that it is just the research object of safety science is pointed out. The types of “zaishi” and their relationships are also discussed, based on which the necessity and feasibility of the construction of safety science are expatiated .The connotation of safety science and the basic approach of the construction are analyzed in a new point of view. Finally，the hierarchy of system of safety science and technology is discussed