Abstract: The lower limb components of humanoid robots, functioning as core load-bearing units, play a pivotal role in achieving lightweight breakthroughs for enhanced system performance. This study focuses on optimizing lower limb structures for 75kg-class humanoid robots through laser additive manufacturing. The research framework partitions the lower limb assembly into calf and thigh segments, establishing topology optimization models with multi-dimensional load cases to precisely simulate actual motion states. Distinct optimization strategies were implemented: a mass-minimization objective function for the calf structure versus a stiffness-maximization optimization algorithm for the thigh component. Iterative computations yielded optimized configurations achieving over 50% weight reduction. Particularly for the calf structure, multi-phase configuration adjustments combined with finite element verification successfully eliminated stress concentration phenomena while constructing a novel structural system that demonstrates both superior mechanical performance and remarkable lightweight characteristics.