Abstract: The formation mechanism of caves in fissured loess at the edge of the loess platform plateau is complex and is closely related to the occurrence of geological hazards such as landslides, which pose a serious threat to the production and life of people in this region. To address the scientific problem of how internal erosion caves are formed in the fissured loess, four sets of experiments on the formation mechanism of internal erosion caves in loess under different initial fissure opening conditions are carried out using the visualisation of the internal erosion caves formation mechanism model of the fissured loess in the South Plateau of Jingyang as a reference. The results show that the development process from the fractured loess to the internal erosion of caves under water flowing erosion can be divided into three stages, including the hydraulic erosion stage, the infiltration and erosion stage and the gravity collapse stage. The formation mechanism of the internal erosion caves in the fractured loess can be summarized as follows: Softening and sputtering, scouring and widening, erosion and cavitation, and collapsing by gravity. The development rate of seepage erosion in the fractured loess is reflected by the sediment quantity and transport characteristics, which can be divided into the rapid erosion stage, the fluctuation reduction stage and the slow and stable erosion stage. The fracture opening, the erodibility of loess and the ultimate groove depth are important factors affecting the formation of internal erosion caves in the fractured loess. The research results provide theoretical support to further reveal the disaster-causing effects of loess internal erosion.