Based on the principle of energy coding, an energy function of variety of electric potential of neural population in cerebral cortex was proposed. The energy function can reflect the energy evolution of neural population with time, and the coupling relationship at sub-threshold and at supra-threshold stimulation. The Hamiltonian motion equation of the membrane potential was obtained under the condition of Gaussian white noise according to neuro-electrophysiological data. The results show that the mean of the membrane potential obtained is just the exact solution of the motion equation of the membrane potential in the previously published paper.