Combined with the design data of an actual foundation pit, PFC (Particle Flow Code) model was developed and micro characteristics such as porosity, displacement vector field and particle rotation were analyzed when a foundation pit with cantilever soldier pile was unstable. The effect of overload, embedded pile depth and the angle of internal friction of soil on instability of the foundation pit were further analyzed. The results indicate that the displacement field under different overload is distinctly different. When piles are shallowly embedded, the foundation pit is apt to be overall unstable and a triangular shaped area of intense shearing and considerable particle rotation appears to form. The soil mass outside the pit compresses at the beginning and subsequently dilates during the failure of the pit. It is shown that factors such as ground overload and the angle of internal friction of soil greatly affect the overturning stability and the slip failure of the foundation pit
