Abstract: A subdomain decomposition-based distributed cooperative coevolution method is proposed in this paper to address the challenges of low load-carrying efficiency, poor load-diffusion performance, and difficulties in lightweight optimization design, which are posed by large dimensions and heavy loads in the concentrated-force diffusion component of the heavy-lift launch vehicle. Specifically, a hierarchical variable-stiffness structural configuration is designed to tailor stiffness properties of the main and auxiliary load-diffusion zones in the concentrated-force diffusion component. Subsequently, linear static analysis is employed to examine stress distribution, while Euler's formula is utilized to evaluate the critical buckling stress of components rapidly. This, in turn, enables the establishment of a high-dimensional, multi-constraint lightweight optimization problem. To efficiently solve this large-scale global optimization problem, guided by the principle of "divide and conquer, cooperate and collaborate", the global problem is first reconstructed into multiple smaller-scale subproblems through rational subdomain decomposition. Thereafter, a distributed cooperative coevolution simulated annealing algorithm framework is proposed, which solves these subproblems concurrently, thereby achieving a simultaneous improvement in optimization efficiency and accuracy. In application to the lightweight design of the concentrated-force diffusion component in the heavy-lift launch vehicle, the proposed method yields a solution that reduces weight by 898.98 kg and improves concentrated-load diffusion capacity by 54.6% compared to the initial design. Furthermore, and significantly, it not only achieves further structural weight reduction but also reduces the iterations by over 60% when benchmarked against existing literature, thus fully demonstrating its advancement and engineering applicability. Ultimately, the research findings can directly support the development of such critical launch vehicle components and are also extendable to other similar complex structural designs, thereby offering considerable theoretical significance and practical engineering value.