Abstract:  At first, this paper proposes an assumption that the formation of loess vertical joints could be divided into two stages. On the basis of the field visits, indoor physical simulation tests, then in combination with loess collapsibility and the mechanical mechanism of producing fissures, the causes of the vertical joints of loess are explored.The results indicate that the formation of loess vertical joints can be divided into two phases: ① the formation of inner fissure, i.e. in the beginning of deposition, the undulation of terrain leads to uneven distribution of loess, then when it rains uneven collapsibility generates, which leads to generate inner fissures ； ② the stage of fissure extension, namely loess continues deposition after the inner fissures have been formed, the increasing of the top pressure makes the top of inner fissure lie in the state of tensile stress, so, when the stress reaches its tensile strength, the inner fissure extends along the vertical direction and then becomes a vertical joint.In the meantime, from survey in site, the rationality of assumption has been proved.