The article deals with the phenomenon of social identity of a group subject from the point of view of the subject approach to small groups in psychology. The problem of studying a small group is the most significant for Russian psychology, and this is due to a number of factors. First, the very study of human development and formation is impossible in isolation from the social environment in which it is included. This inclusion is not always based on an individual’s conscious choice, but is sometimes simply due to the time and place of birth. Secondly, the worldview of a person often identifies the concept of personal and social, in the Russian mentality «I» is associated with the social groups in which it consists (educational, professional, sports, etc.), so the problem of studying small groups is especially relevant in our time. In addition, social and economic instability increases the need to join and receive protection from group membership. The article emphasizes the lack of development of this topic in domestic and foreign psychology. The author describes the developed method of diagnostics of social identity of a group subject and its application in groups of cadets of different training courses in the educational organization of the Ministry of internal Affairs of Russia. The analysis of the obtained psychodiagnostic results and the General conclusion about the effectiveness of a specific technique, as well as about the category of social identity of a group subject, depending on the direction of training and the duration of training in an educational organization of higher education.