Abstract: When the weave of earthquake spreads out in the earth, it can radiate the infrasound. The kind of infrasound is called as earthquake infrasound. The frequency of infrasound is under the 20 Hz, it's magnitude is declined slowly, and at the same time it spreads far. The infrasound can spread several kilometers long in the air (from the earth's surface upper to the 50 kilometers). The infrasound can be detected after circling the earth several weeks. Because of these characters, the infrasound has the good opposite connection with the earthquake. As a rule, the abnormal infrasound signal can be detected beyond the magnitude 7 of earthquake in the world. Using the good capabilities of computer and the more and more perfect technologies of signal analysis to find out the inside qualities of infrasound abnormal signal before the earthquake, which will provide the important value for forecasting the earthquake later. Using the discrete Fourier transform and wavelet theories, the abnormal signal is found and it's range of frequency is about from 0 Hz to 4×10^-3 Hz after the abnormal signal is discrete, at the same time the abnormal signal is symmetrical and regular in higher scale and the abnormal signal is broken in lower scale.