Abstract: To address the excessive peak force and low energy absorption efficiency of conventional thin-walled energy absorbers, this study proposes a biomimetic multi-branch thin-walled energy absorber inspired by the load-dispersing and anti-buckling characteristics of the multi-rib topology of cactus stems. First, the energy absorption mechanism of the proposed structure was elucidated based on the super-folding element theory, revealing that the synergistic load-bearing effect between the central stem and peripheral ribs enables uniform impact load transfer and progressive energy dissipation. Subsequently, a finite element model was established in ABAQUS to systematically investigate the deformation mode, crushing force response, and energy absorption characteristics under a low-speed impact condition of 4 m/s. A multi-objective optimization framework combining a Kriging surrogate model and the NSGA-II algorithm was then developed to obtain the optimal structural parameter combination. The results show that, compared with the conventional square energy absorber, the optimized biomimetic multi-branch energy absorber exhibits superior crashworthiness under low-speed impact, characterized by reduced peak force, enhanced energy absorption, and improved load-bearing capacity. Specifically, the peak crushing force was reduced by 23.8%, the mean crushing force increased to 79.98 kN, and the specific energy absorption increased by 124.5%. In addition, the Johnson–Cook constitutive parameters of 3D-printed 6061 aluminum alloy were calibrated through laser powder bed fusion additive manufacturing and orthogonal experiments. The accuracy of the finite element model was further validated by quasi-static compression tests, with the relative discrepancy between the simulation and experimental results below 7%. This study provides a theoretical basis and technical support for the biomimetic design of automotive energy absorbers.