Abstract: Based on previous studies, rock thermal physic and wicking action theories are used to examine the effect of the alternative change of heat and water on the slaking of redbeds, especially the rock structure before they are impacted by water and the alternative change of heat and water to the slaking. The results suggest that the alternative change of water and heat is the engine to the mudstone slaking. During the alternation, the fissures are built and the rock structure is created for the water permeation by the thermal stress within 0.3～0.4 m of mudstone from the surface. The effective slaking depth is about 20～30 cm/a and the slaking process can be separated into five parts: excavation, thermal effect, water action, wetting and drying cycle (time effect) and peeling off. The slaking phenomenon is caused by the coactions of water, heat and force.