Transcutaneous Electrical Nerve Stimulation (TENS) attracts extensive attention in neuromodulation due to its minimal invasiveness. Conventional TENS uses non-invasive surface planar electrodes attached to the skin for delivering the stimulation current to modulate neural circuitries. Previous studies have shown that using these surface planar electrodes in TENS therapy may cause stimulation-induced skin irritation. In order to validate the cause and solve this problem, a bioheat transfer study in a realistic human spinal cord model was built based on finite element method (FEM) and a spiked electrode is proposed to avoid skin burning by taking into consideration both the electrical and thermal properties of the layered skin. Results show the peak temperature of using the spiked electrode for stimulation is around 37.1 °C, much lower than using the planar electrode (51.6 °C). Moreover, in contrast to the traditional surface electrodes, the peak necrosis fraction of using the spiked electrode is 0.02, indicating that the whole biological model is thermally safe and does not cause skin irritation.