Abstract: Road collapses occur frequently, which is a large threat to the city safety. The collapse is initiated by the ground cavity caused by the internal erosion around buried broken pipelines. In view of this problem, a model test method is presented to study the characteristics of the internal erosion due to broken pipelines in gap-grading sand. The process of erosion could be divided into three stages of rapid erosion, steady erosion and convergence stage in terms of the mass of eroded soil. Loose area and cavity are produced due to deformation of eroded soil and expand similarly to the process of erosion. The soil erosion and final erosion states under different conditions are compared, which reveals that the soil saturation and water level can influence the erosion amount and cavity state by affecting the seepage force. Besides, the initial soil grading determines the content of fine sand which can migrate away, affecting the erosion degree and cavity area. Finally, the larger pipe defect causes more migration pathways and increases the soil loss when the defect shape allows the fine particles to migrate into the pipe merely, but it has little influence.