Abstract: Due to the large intensity of back silting and long-term idleness of foundation trench, back silting will threaten the safety of immersed tunnel. Relying on Shenzhen-Zhongshan immersed tunnel project, the change rules of back silting with time and space in the trial trench tests are summarized. A numerical model is established to study the influences of back silting on the stability of foundation trench slope. The factors affecting the local stability of back silting are deeply analyzed. The results show that: (1) The horizontal distribution of back silting is obviously different, that is, the amount of back silting at the bottom, north slope and south slope decreases in turn. The vertical distribution of back silting is uneven, and the thickness of back silting from top to bottom decreases. The amount of back silting varies greatly through out the year, large in flood season but small in dry season. (2) The stability of foundation trench slope is mainly controlled by the back silting, and the stability of back silting can be significantly improved by desilting with slope. The safety factor of back silting decreases with the increase of thickness of back silting, desilting gradient, slope height, slope gradient or the decrease of shear strength of back silting.