The scientific realism is the epistemological foundation of the western international theory.From 1920 to now,there have been three times of academic debate.They are the controversies between Realism and Idealism in 1930s,Traditional Realism and Scientism in 1960s,Neorealism and Neoiberal Instiutionalism,constructivism in 1980s.Whatever their arguments are,the trend of change is scientification,This essay tries to analize the process of change,so as to grasp the essence of the western internationnal relation theory.