A flexible mechanical joint for human exoskeleton based on low-melting-point alloy is proposed for the first time. With the liquid-solid phase change capability, this unique joint can easily switch between its flexible and rigid states. Such mechanism allows the fabricated human exoskeleton arm to conveniently and effectively perform the powerful lifting action. Conceptual investigations disclosed that metal-based mechanical joints provided excellent moving flexibility and large loading capacity. Comparative measurements demonstrated that the phase change joint responded much faster and could support heavier loads than that of the conventional paraffin-based joint. This method opens a soft mechanical joint for human exoskeleton in future civilian use.