Abstract: The Tiantai Landslide is a soil one of extra large-scale occurred in the region of gentle slope and gently dipping formation. The main character is slipping in multi-stage and mass-by-mass. It's horizontal displacement is the largist in the front and reducing towards the back. Five secondary deforming patterns are summarized according to the varied platform topography with troughs and ridges, the essential conditions producing the multi-stage and mass-by-mass sliding is analyzed, and genetic model forming the mass-by-mass sliding by water pressure is established.