Lattice strain plays a critical role in controlling catalytic performance. However, few experiments or theoretical works have been reported regarding the strain–activity relationship in the hydrogen evolution reaction (HER). Herein, we have prepared platinum–skin-truncated octahedra with the formula CoPtx. Among the prepared nanocatalysts of varied composition, the CoPt2 nanocatalyst exhibits an extremely small overpotential of 17 mV in acid solution at −10 mA cm–2, which is obviously superior to commercial Pt/C (30 mV). Furthermore, the overpotential of the CoPt2 nanocatalyst is only 20 mV in alkaline solution, which is about a third of that of commercial Pt/C (53 mV). The experiments and density functional theory calculations reveal a “volcano”-type relationship for the strain-HER catalytic activity relationship for the first time. The theoretical calculated “volcano” relationship gives a clearer scope of the best strain toward further investigation on higher HER activity.