We proposed a novel pain intensity assessment method based on changes in peripheral arterial stiffness index β, which responds to peripheral sympathetic nerve activity. β is calculated from continuous arterial pressure and plethysmogram per beat based on a log-linearized peripheral vascular viscoelastic model. The validity of the proposed method was examined by comparing the estimated normalized arterial stiffness index βn and the subjective pain assessment index (numeric rating scale: NRS) as an index of the pain intensity when electrical stimulation is applied to the skin of healthy subjects. The results show that the maximum amplitude of the current (Imax) has a linear correlation with βn (R2 = 0.77, p < 0.05). In addition, Imax and βn are correlated with the sigmoid-transformed NRS (Imax vs NRS: R2 = 0.91, p < 0.01; βn vs NRS: R2 = 0.92, p < 0.01). Therefore, we concluded that the proposed method can objectively evaluate the intensity of pain.