Layered deposited-soil slopes are widely distributed in mountainous terrain. The rainfall-induced instability of layered deposited-soil slopes is not only controlled by the unsaturated infiltration process but also by the seepage-induced internal-erosion process within the deposited soils. In this paper, the main physical processes within two-layer deposited-soil slopes under rainfall infiltration are summarized, and a coupled seepage-erosion finite element model is established to analyze the interactions between the rainfall infiltration process and the internal-erosion process within layered deposited-soil slopes. This finite element model was validated by simulating the coupled seepage-erosion process in a one-dimensional layered soil column. Then, serials of two-dimensional coupled seepage-erosion simulations were conducted to investigate the rainfall-induced seepage-erosion patterns, as well as their impact on the stability evolution of the layered deposited slopes. It was shown that the rainfall-induced seepage-erosion accelerate the water infiltration into the slope and facilitate the generation of subsurface stormflow near the layer interface, which will weaken the soils around the layer interface and accelerate the slope failure process inevitably. Special attention should be paid to the rainfall-induced seepage-erosion effect when evaluating the stability of layered deposited-soil slopes.