Abstract: A penetration arc sound experimental system in MIG welding was developed.By analyzing the characteristics of the spectrum of arc sound signal with projected transfer and spray transfer,the procedure of arc sound frequency-band energy extration was designed.Firstly,wavelet transfer is utilized in elimanating noises of arc sound in welding process.Secondly,wavelet packet frequency-shift arithmetic is used to extract arc sound frequency-band energy,which eliminates the frequency aliasing phenomenon caused by the interval sampling in the classical wavelet packet decomposition.The last,frequency-band energy is calculated and the characteristic vector is constructed.The experiments show that the extracted frequency-band characteristics can better reflect the weld penetration.