Different from the light distribution along the propagation direction in optical thin-film induced by nanosecond pulse or continuous wave, the one irradiated by femtosecond laser pulse is an unsteady process, and can not be solved directly by characteristic matrix of coatings. The model is setup by means of multiple-beam interferometry in time domain, and simulated with the parameters of ZnS material after that. The result reveals that, to single layer reflecting film, the reflectance is proportional to the pulse width, and equals ultimately to what illuminated by continuous wave. Furthermore, under the same pulse width, the reflectance decreases with the increase of the film thickness. The distinction of light intensity distribution induced by ultrashort laser pulse and continuous wave is obviously, as well as the increase of the film thickness. In detail, the curve of intensity distribution in the film irradiated by continuous wave flunctuates with the fixed amplitude. However, it grows up gradually under the ultrashort laser pulse.