The detection of somatosensory evoked potential (SEP) has been an important approach to determining the function of spinal cord and peripheral nerve. Transepidermal Electrical Neuromuscular Stimulation (TENS) on the posterior tibial nerve for SEP is one of the applications and can improve the effectiveness. This paper theoretically studies the distribution of potential field and current density field for human tissue under TENS. A mirror method is used and the analytical solution of the potential distribution is obtained.