Arc sound signal in MIG welding contains plenty of welding information, which is an important signal source of weld penetration. Aiming at on-line monitoring of weld penetration, the relationship between arc sound signal and penetration is analyzed in time-frequency domain through a reasonable selection of sample data. The results show that the spectrum of arc sound signal and penetration status are highly correlative. The changes of penetration can accurately detected by variation of signal energy from 1500 Hz to 4500 Hz in welding process. The above experiments verify the feasibility of monitoring the penetration through arc sound signal and provides theory and application foundation for penetration adaptive control.