Electrical nerve stimulation is an effective method for certain treatments by affecting the central or peripheral nervous system. The instability of electrical nerve stimulation is a critical problem in clinical practice. It is a widely held view that the mechanism of the instability is that the electrical stimulation disturbs the membrane potential of nerve axons. However, due to the lack of a computable macro model for electrical nerve stimulation, it is difficult to effectively study the specific impact of its membrane potential disturbance on electrical stimulation for a long time. Based on the previously proposed circuit-probability theory, this study qualitatively analyzes the instability of electrical nerve stimulation to effectively research the influence of membrane potential disturbance on electrical stimulation. The results show that the current-instability curve of animal experimental data and qualitative simulation is highly consistent, which further indicates that the circuit-probability theory might explain the membrane potential disturbance caused by electrical stimulation and has instructive significance for the practical application of electrical nerve stimulation.