In this paper, the basic damage mechanisms and the primary reasons for the property fluctuation of recycled aggregate concrete are investigated experimentally. By a comprehensive literature study and systematic laboratory tests, the interactions between the old and the new interfaces in recycled aggregate concrete are analyzed. In particular, the damage initiation and evolution mechanisms on the old and the new interfaces are studied in details. The essential factors affecting the fluctuation and its extent are investigated from the point of view of material sciences. The present results imply that the qualitative and quantitative changes of the old and the new interfaces during the loading process induce a notable fluctuation of the mechanical and the physical properties of recycled aggregate concrete. To reduce the fluctuation and improve the mechanical properties of the recycled aggregate concrete, effective controlling and processing measures are suggested and discussed.