A theoretical scheme of coherent excitation atoms are proposed to explain the essence of supernarrow resonant effects, which are produced by using the mode-locked laser pulse train exciting the atoms. The expressions of the atomic density operator ρA(t) and of the signals of spectra are derived and the relevant ourves are given. The new results for the supernarrow resonant effects are also predicted. The theoretical scheme can be extended to the application for multiphoton processes.