Pit-in-pit foundation pit is a more complex and not yet fully studied form of foundation pit. In order to study the internal force development law of the supporting piles of the inner pit during the excavation of the foundation pit, field tests of the internal force of the support piles during the construction process were conducted. The bending moment of the supporting pile is calculated by collecting the stresses in the longitudinal reinforcement of the pile, and the development law of the bending moment of the supported pile during the construction process was analyzed. The experimental results were fitted by finite element numerical calculation, the trend of the bending moment of the inner pit support pile was studied when the depth of the outer pit excavation and the width of the platform were varied. The results show that there is a close relationship between the pit toe factor ( the width of the platform / the excavation depth of the outer pit ) and the variation of the maximum bending moment of the inner pit support pile. The function curve of pile maximum moment growth rate of pit toe factor is given by fitting. The pile maximum moment growth rate can be considered as the safety factor in the design of pit retaining pile in pit-in-pit foundation pit.