In this paper the semi-classic theory of coherent excitation of a two-level atom was modified and applied to a two-level atom with a hyperfine structure. The im portant result of calculation is as follows: when the exciting laser power is la rge enough, due to the power broadening effect, a narrow-band laser can excite a two-level atom with a hyperfine structure effectively. So it is not necessary t o use a broad-band laser to guarantee excitation efficiency for an atom with hyp erfine split levels.