Abstract: Based on the field survey and lab experiment,this paper analyses the forming factors of loess caves in west Shanxi.Research shows that the combined function of various external and internal factors is the main origins of loess caves in west Shanxi.The main factors include loess quanlity,loess microstruture,joint and fracture,loess layer thickness,hydro-geological and climate,topography and morphology conditions.The loess quality and microstructure is the interior factors of loess caves formation.The loess layer,joint and fracture present interspace and physical base for loess caves.The upland water and ground water are the motive power of loess caves formation.The topography and morphology conditions control the area distribution of loess caves.