Action potential dynamics of nerve fibers is related to the information conduction and code among neurons. Nowadays, traditional electrophysiologic approach still could not fast probe the action potential dynamics simultaneously at multiple positions on nerve fibers. Nonlinear optical second harmonic generation combined with mathematical modeling is used to study the characteristics of myelinated nerve fibers. The detected sensitivity of the method is analyzed at first, and then, the information changes induced by the changes in morphology including axonal diameter and myelin thickness on a myelinated nerve fiber under a compression are investigated. The results demonstrate that changes in membrane potential induced by a compression may be fast probed via second harmonic generation. Furthermore, the action potential conduction is blocked when severe demyelination happens on the nerve fiber. It is shown that optical second harmonic generation is a potential tool to evaluate the injured states of nerve fibers.