Single-molecule pulling experiment provides a powerful tool for a fundamental understanding of the structure,dynamics,and function of biological systems,such as the mechanical properties of protein unfolding [1].In experiments,pulling forces are applied to ends of the molecule,forcing the rare rupture event to occur more frequently.Generally,one expects that the average rupture time would decrease monotonically with increasing force.However,it was observed in experiments that the rupture time may show an abnormal increase for low forces,followed by a maximum and then decreases at higher forces[2].Although a few phenomenological models were introduced to address such an interesting behavior,a deep understanding of the underlying physical mechanism is still lacking.