Humanoid robots mimic human-like movements by emulating the human body’s morphology, with a single lower limb typically configured with five or more degrees of freedom (DOF) to execute complex motions. However, vibration coupling induced by multi-motor coordination can lead to forced vibrations or even resonance in structural components, significantly increasing the complexity of vibration analysis. This study employs the finite element method (FEM) to construct a single lower limb model of a humanoid robot. Modal analysis is performed to extract the lower limb’s natural frequencies and mode shapes, while harmonic response analysis evaluates amplitude variations at critical regions under different excitation frequencies. Further optimization via material selection is conducted on robotic components, targeting modifiable regions—including hip joints, thigh linkages, and calf structures—to mitigate vibrational behavior. The results provide actionable guidance for enhancing vibration suppression capabilities of lower limb, offering hardware-level support for high-speed motion stability and operational precision in complex environments.