Self compacting concrete (SCC) filling layer is core structure of China rail track system (CRTS) ? type ballastless track. Construction quality, service performance and durability of CRTS ? ballastless structure are affected by stability of SCC for filling layer. In this study, the stability of SCC of filling layer is researched at three levels as paste, mortar and concrete by theory and experiment. Evaluation indices including bleeding (��), surface bubble rate (��), thickness of paste (��paste) and thickness of surface mortar (��) are proposed based on the theoretical calculation and analysis. The threshold viscosity of paste 0.394 Pa·s and mixture satisfied area are obtained at paste level based on the relationship between viscosity and ��, �� of paste. The mixture satisfied area was defined at mortar level under criterions of maximum value of ��paste and slump flow. Optimal range of gap between neighboring aggregates (��ca) 12.4 mm~14.1 mm is chosen by flow ability, passing ability, stable ability of SCC. These research results will help to further understand the stability of SCC.