The characteristics of signal transmission speed in the condition of superluminal group velocity are discussed. When an amplitude modulated sinusoid signal is sent into mismatched transmission line groups at some special carrier frequencies,its group velocity of envelope is superluminal. The speed fluctuation according to the length changes of the line is verified. The group velocity of the signal envelope in the forbidden band is one to two times faster than the speed of light(c)in vacuum. Even negative group velocity occurs when the total length of the mismatched line group is long enough such as 179.8 m. According to the experiment,differences between superluminal group velocity in certain condition and the light speed in general propagation exist in not only the speed values but also the speed fluctuations. The experimental result reveals that the characteristic of superluminal group velocity propagation speed is obviously different from that of electricity and light in normal propagation.