In areas with strong earthquakes, the triggering rainfall threshold for debris flow initiation decreased dramatically just after the quake, but will increase gradually during the subsequent raining seasons, until the level before the quake. The channel width and particle size in the source area of debris flows in catchments decreased dramatically during a strong earthquake due to the supply of finer material in channels by co-seismic landslides. Debris flows in these areas are initiated by run-off water in channels and the presence of loose materials and narrowed channels leads to a significant decrease of the critical rainfall threshold for debris flow initiation just after the earthquake. The gradually coarsening of the material and widening of the channels during the subsequent raining seasons by debris flows and flashfloods will rise again the triggering threshold. Based on these important principles, a revised prediction model on debris flows is proposed, which incorporates the effect of channel width and the particle size of channel materials. The new prediction model was successfully validated on a group of debris flow events in Dechang, Sichuan Province, China. The new prediction model was also able to successfully simulate the change of the triggering rainfall threshold before and after the Wenchuan Earthquake in the Wenjia Gully, Sichuan Province, China, where five debris flow events happened during three subsequent rainy seasons after the earthquake. It is suggested that the new prediction model on debris flows (can also be applied) is also valid in non-seismic areas where significant loose material is available supplied by large landslides.