The influence of pulse distortion on superluminal group velocity in an anomalous dispersive medium is researched. The result shows that superluminal group velocity does not disappear even if there is no distortion for light pulse. But the group velocity of a pulse is not equal to the signal velocity. An effective velocity of information transfer is defined based on the information theory, and is applied to explain the experiment that a negative group velocity was observed by Wang, Kuzmich and Dogariu (WKD). Calculation of the information content of input and output pulse shows that the information content of output pulse will decrease due to diffraction of light wave and noise of photon. The effective velocity of light propagation is not greater than the light velocity in vacuum.