Abstract: A method of planar laser induced fluorescence (PLIF) technology combined with digital image processing technology was proposed to measure the free falling film thickness on a vertical testing section. The time sequences of film thickness of the film flow at different Reynolds numbers were obtained in the experiment. The frequency structure was analyzed on the base of the function which was used to calculate the power spectrum density in MATLAB. The results indicate that the thin-film wave flow has the multiple frequency characteristics. The solitary-type waves contain the largest portion of moving liquid mass. The wave amplitude depends strongly on the Reynolds number. The wave amplitude increases with the Reynolds number at small Reynolds number. The amplitude remains stable at medium Reynolds number. The appearance of the solitary-type waves at high Reynolds number leads to sharp increase of the wave amplitude and enhancement of the wave instability.