A nonlinearity model of the detector′s power-current (P-I) characteristics is established based on the theories and experiments in respect to photo-electric conversion efficiency. The model is used to analyze the detector′s photo-electric convertion ability in both linear and nonlinear cases. With the theories and experiments, the frequency response of the visible light communication (VLC) system when photodiode becomes saturated is studied. It proves that although the afterglow of fluorescent powder cannot be modulated in high speed, it can decrease the frequency response of the detector. The detector′s saturation due to the high optical power has an effect on the frequency response and the bandwidth in VLC system.