To balance the conflict of displacement and axial force of the support system and adjust the length of the internal support, a theoretical calculation model is developed for the dynamic adjustment of single and multiple internal supports of foundation pits based on the plane strain problem. A method for calculating the axial force of the internal support and the displacement of the diaphragm wall caused by the adjustment of the internal support is derived. In addition, a method for determining the dynamic adjustment range of the internal support is proposed. The theoretical method is validated through engineering examples. The findings reveal that the internal support requires adjustment within a reasonable range and the theoretical method presented in this study can be used to determine the appropriate range of dynamic adjustment of the internal support. However, when the calculated minimum adjustment value exceeds the upper limit value, i.e., when the dynamic adjustment of internal support fails to meet the safety control requirements, the foundation pit support scheme is unreasonable and must be redesigned.