The vertical deviation of retaining piles often occurs in deep foundation pit engineering, which would lead to the invasion of the main structure boundaries. In order to ensure the size requirements of the main structure, it is necessary to cut the invaded parts. After the pile is cut, its bending moment bearing capacity will inevitably decrease. For the calculation method of bending moment bearing capacity of circular reinforced concrete piles after partial excavation, the calculation method of bending bearing capacity of normal crosssection of circular reinforced concrete piles after partial excavation is established based on the second approximation method. The influence of partial excavation of retaining piles on the safety of retaining structure system of foundation pit is analyzed based on practical engineering. The results show that: (1) The cutting of longitudinal bars and the reduction of the crosssectional area of the pile, due to partial removal of pile, are the main reasons for a rapid decline in the flexural strengthening of the pile. (2) In a practical engineering, the diameter of retaining pile is 800 mm, and after cutting 18 cm thickness of those parts beyond the boundary (6 main ribs being cut), its flexural capacity of normal crosssection is decreased to 55%, which could not meet the requirements of safety support, hence reinforcement should be performed.