TMS (Transcranial magnetic stimulation) is an effective method of cortex stimulation for the treatment of neurological diseases. Here we change the magnetic stimulation into external electric field form and add it in a minimum neuron model. Through detailed analysis of phase plane and bifurcation type, we obtain the nonlinear kinetics law of the firing pattern change derive from external electric field. Further we propose the idea of external electric field affect the different ion currents competition in sub-threshold. Finally the effect of external electric field on action potential initial dynamics was studied by which we can explain the physiological mechanism of TMS's effect on neural coding.