Although some researchers had discussed the properties of applied psychology, we were still confused by the question what is applied psychology? To answer it systematically, the present paper adopted the Qian xuesen’s theory of subject structure which was retrieved from the history of physics. There are two debating theories explaining the basic attributes of applied psychology, i.e. the pure application theory and the independence theory. The former argued that applied psychology just directly implicated the knowledge of basic psychology into practical realms; however, the latter insisted that applied psychology have its own indispensable properties. The pure application theory dominated the history of psychology; however, it left some phenomenon not explained. For example, the basic psychology has not had the all knowledge about human beings’ psyche. In fact, the application was a re-creative process which was distinguishable from process of the pure study. So more and more psychologists believed the independence theory be correct. As is well known, it lacked a compellent theory explaining why the independence theory was correct. Qian xuesen who was an applied scientist putted forth a theory that there are three distinctive groups of subjects within the field related to physics. They are the pure science, the engineering science, and the technical and engineering practice. Qian xuesen argued that almost all subjects have these three groups. According to this theory, the present paper analyzed the history of psychology, and demonstrated that psychology also had the three groups, i.e. basic psychology, applied psychology (also called psychotechnology), and technical and engineering practice of psychology. Applied psychology, as a historical product, focused on studying key problems proposed by the practice of psychology via scientific methods. This definition determined the basic properties of applied psychology. The present paper analyzed the aim, contents, and methods of applied psychology which were distinctive from the other fields related to psychology. Applied psychology is produced by the demands of practice. Its branches are shaped by these demands and its own history. Its methodology emphasized the role of experience of researcher community. The possible contribution of the present paper was that relating the Qian xuesen’s theory to the literature to explore the properties of applied psychology. Some discussions concerning the developmen