1. A voice activity detection (VAD) device, comprising: a background analyzing unit adapted to analyze background noise features of a current signal according to an input VAD judgment result, obtain parameters related to a background noise variation, and output the obtained parameters; a VAD threshold adjusting unit adapted to obtain a bias of the VAD threshold according to the parameters output by the background analyzing unit, and output the bias of the VAD threshold; and a VAD judging unit adapted to modify a VAD threshold to be modified according to the bias of the VAD threshold output by the VAD threshold adjusting unit, perform a background noise judgment according to the modified VAD threshold, and output a VAD judgment result.