The Voice Activity Detector(VAD) of G.729B in VoIP can only work well in the high SNR, while in the low SNR or with a strong background noise it works poorly. To tackle this problem, the VAD2 mechanism of AMR is researched in detail and the possibility of introducing it into the G.729B is discussed. At the same time, some optimization work is also done. Through Matlab simulation, the optimized VAD2 is proved better by comparing it with the VAD algorithm of G.729B in both the high and low SNR, and its detection time is shortened greatly. So the optimized VAD2 can be used in G.729B.