From the sociological point of view, virtual learning community is a kind of special community, and social interaction in the virtual learning community is the key to enable the teachers and students to "live" together and thus forms a virtual learning community, who separated spatially and temporally. Using the content analysis method and participant observation method, this paper analyzed the interaction structure, including the space structure, role structure, content structure, behaviour structure, and time structure of social interaction, based on the case study of "SCNU online" -- one of typical cases of virtual learning communities in the field of modern distance education in China. It concluded that:First, there are usually three kinds of social space in virtual learning communities where learners communicate with others, namely individual space, group space and public space. While in the group space, there is a kind of new organization form: "curriculum-organization", which has some differences with tradicational class and grade organization in the existence way, instruction flexibility, population scale and structure, member's sense of belonging and identification, the organizational structure, etc.Second, the interaction frequency among different learners has large differences in virtual learning communities. According to the absolute value of the interaction frequency, distance learners can be divided into four kinds: active participants, general participants, rare participants and spectators, of which the active participants are in the minority, the rare participants and spectators are in the majority. The curriculum-interaction-population-density, a dynamic value in the time dimension, usually less than the curriculum-population-density in quantity dimension, should maintain in an appropriate value sector, not too high and too low, and should approach to the curriculum-population-density in quantity as close as possible.Third, virtual learning communities become an important position for distance learning organizations and tutors to provide various academic and non-academic student support services. The academic student support services include cognitive