The frequency characteristics of the Daubechies wavelet series are analyzed. It is pointed out that there is a boundary problem for most frequency band boundary frequencies, the frequency band energy leaps at the left and right sides of the boundary frequencies, and the leap directions are reverse. The veracity of the signal feature extraction is badly affected by the boundary problem. Therefore, a frequency band comparison method is put forward. The rule for judging the boundary frequencies is given. The boundary frequencies can be accurately identified with the method.