The interaction between radiation and two-level atom was investigated by semi-classical theory and quantum theory respectively. The fidelity of the two-level atom's quantum state was written out. The influences of the radiation intensity in the case of semi-classical theory and the photon number of the radiation in the case of quantum theory on the fidelity were discussed for different atomic initial states. It is found that the fidelity of quantum state is independent of radiation intensity when the atom is initially in a superposition state and the atom interacts resonantly with the radiation field in semi-classical theory.The atom's quantum fidelity evolves periodically with time,and its frequency is modulated by radiation intensity. Furthermore,it can effectively change quantum information fidelity when the appropriate detuning.is selected.