Landslides induced by heavy rainfall or earthquake sometimes result in the blockage of river flow to form barrier lakes. Relevant data are important for understanding the formation and stability of landslide dams; the inventory of the fundamental landslide-dam data is hence essential. According to the statistics of recorded cases, landslide dams often breach in a short time. As a consequence, it is often difficult to collect comprehensive information of a landslide dam with a short life after dam breach. It is, however, possible for case reconstruction of a breached landslide dam through various methods. This study made use of the particle-flow simulation code PFC3D to model the processes of the formation and evolution of landslide dams induced by heavy rainfall. In the past, the common approach to trigger a landslide's initialization by the particle-flow simulation is to substantially reduce the inter-particle frictional coefficient. However, this approach may lead to incorrect interpretation of the run-out process of the landslide mass and the dam formation. Different from the common approach, the present work proposed a new method by adding a seepage-force term on all particles within the sliding mass under the ground water table. Because of the applied seepage force, the landslide mass can lose its stability and starts to move. The simulated results demonstrated that the proposed approach is a simple and feasible approach to model the trigger of a landslide owing to heavy rainfall. This approach was applied to an actual case of landslide dam with a short life (The 2009-August landslide dam in the Tai-Ma-Li River).