Transcranial magnetic stimulation (TMS) is a kind of brain stimulation method of producing magnetic field at the designated area of brain employing electromagnetic coils. The principle of TMS is to apply an electric field which is generated through the electromagnetic induction to neuron, thereby influencing the excitability of neuron. Though it has been used for decades, its underlying mechanism, i.e., how TMS induction electric field changes neuronal excitability, is still unknown. To address this problem, we establish a minimum neuron model under action of TMS induced electric field, analyze the mechanism from the viewpoint of action potential initial dynamical mechanism which has been proved to be the decision factor of neural coding in previous studies. Through phase plane and bifurcation analysis, we reveal the dynamical mechanism of different firing patterns of neuron. Finally, we find that the physiological basis of different excitabilities under action of TMS induced electric field, which is the different outcomes of competition between ion currents of neuron with different kinetic behaviors in sub-threshold potential.