In order to investigate the mechanism of bending stiffness reduction in the pipe-combination (PC) piles, Abaqus was employed to establish a numerical finite element model considering the interaction between lock joints. The variation in bending stiffness of PC piles was analyzed under different factors such as lock joint friction coefficient, pile head constraints, and combination methods, and the influence of neutral axis displacement on the stiffness of PC piles was analyzed through theoretical derivation. The results show that increasing the friction force between lock joints can effectively mitigate the bending stiffness reduction effect of PC piles. When the lock joints are fully welded, the stiffness reduction coefficient of PC piles increases to 0.83. The presence of lateral support at the pile head can significantly reduce the pile’s 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. For engineering applications, it is suggested that the steel sheet piles should be arranged in opposite directions; the lock joints should be welded, and lateral support should be provided. Furthermore, it is recommended to match one steel pipe pile with one steel sheet pile, or use C9, C-T, or L-T type lock joints to satisfy soil retaining and water isolation requirements while ensuring foundation pit safety.