Voltage fault traveling-wave after transmission line faults is a wide frequency band signal. It brings about difficulties for traveling-wave based protection to be applied into power systems because capacitor voltage transformer cannot transmit wide frequency band voltage traveling-wave effectively. The mathematic tool—wavelet transform is utilized， and its multi-resolution signal decomposition characteristic is used to decompose the voltage fault traveling-wave into different frequency band signals. Besides， the polarity relationships among the initial voltage fault traveling-wave in different frequency bands are studied. The studied results show that the initial polarities of voltage fault traveling-wave in different frequency band signals are consistent， and the initial polarities of power frequency components in voltage fault traveling-wave are consistent with the wave front polarity of the initial voltage fault traveling-wave. This conclusion provides theoretic basis for new traveling-wave based directional comparative protection principle.