The deposition morphology of a landslide dam determines the position of initial breach and sequentially affects the direction of lateral erosion. Twenty-seven discrete element numerical simulations were carried out to investigate the deposition morphology of landslide dams concerning three factors, which include the valley shape, the valley bed inclination and the landslide velocity. The simulation results show that the valley shape and the landslide velocity affect the dam morphology in both transverse and longitudinal directions, while the valley bed inclination mainly affects the longitudinal morphology of dams. Further, a flume experiment was carried out to investigate the specific effects of the dam morphology on the overtopping erosion of landslide dams. The results of experiment show that for the dams in different deposition morphologies and with different initial breaches, the breach widths and the total sediment discharges induced by overtopping erosion are different.