In order to investigate the mechanism of stiffness reduction in PC pile construction method, the Abaqus was employed to establish a numerical analysis finite element model considering the interaction between lock joints, the variation in bending stiffness of PC piles under different factors such as lock joint friction coefficient, pile head constraints. Combination methods was compared, and the influence of neutral axis displacement on the stiffness of PC piles through theoretical derivation was analyzed. The results show that increasing the friction force between lock joints can reduce the stiffness reduction effect of PC piles, and the stiffness reduction coefficient of PC piles with fully welded lock joints can be improved to 0.83. The presence of lateral support at the pile head can significantly reduce pile lateral displacement and the stiffness reduction effect. The bending stiffness of PC piles decreases with an increase in the number of steel sheet piles inserted between the pipe piles, resulting in a smaller stiffness reduction coefficient. In order to meet the need of retaining soil and isolating water and ensure the safety of the foundation pit, it is suggested that the steel sheet piles should be arranged in opposite directions, the lock buckles should be welded, and lateral support should be provided, one steel pipe pile should be matched with one steel sheet pile or only matched with C-9, C-T, L-T lock in the project.