Abstract: The geomorphy character of Qian'an mud forest is very special. After ages of erosion, loessial soil changed into group of pillars in dry environment. Because it looks like hoodoo, which is a kind of karst topography, it is called "mud forest". There are several kinds of explains for its formation. The paper indicates that the prime reasons are geomorphy, geology tectonism, groundwater and wind erosion. Through the research we concladed that besides above geological dynamic actions, soils' grain size and physicochemical character are dominant factors in mud forest forming.