The current natural environment is unpredictable with heavy rains and floods happening from time to time. In order to ensure the safety of underground workers, infrastructure, and rail transit, a mobile and expandable underground flood control wall was optimized and designed. The use of mortise-and-tenon structure splicing and modular extension design effectively increases the stability and useable area of the flood wall. The finite element analysis software was used to simulate and study the force of each component under combined loads, such as static water, dynamic water, and impact, to analyze the stability performance of the assembly such as anti-sliding, anti-tilting, and internal stress. The verification results show that when the designed and studied underground flood control wall equipment is fully deployed, the maximum stress is 220.762 MPa, and the maximum offset distance is 32.334 mm, which are all within the safe range. It provides innovative ideas for the optimization of the related flood control wall structure.