It is of great significance that monitoring of weld seam penetration for the welding process quality control and automation production.A pulse variable polarity plasma arc (VPPA) welding system was established and its arc voltage for different penetration status was decomposed into different frequency bands using wavelet transformation.The each frequency energy was calculated and feature information of penetration status correspond to frequency energy was extracted.The results show that weld penetration status is reflected more accurately to high frequency band energy for arc voltage of pulse VPPA welding.There are significant sudden changes in the band energy at the moment when the changes of weld penetration status happen and it can be detected on line for pulse VPPA welding process.A new method for real time control of pulse VPPA welding penetration is proposed.