Abstract: Acoustic signal is an important parameter in laser welding process, and the understanding of mechanism of the laser acoustic signal is not deep enough. A thorough research on the mechanism of sound source and keyhole profile during laser deep penetration welding process were carried out. By considering the Helmholtz effect in acoustics, the relationship between the morphology of the keyhole and the intensity of the sound signal and the resonance frequency was obtained through the acoustic-electric analogy. CCD image acquisition system was used to obtain the actual shape of the hole, and the rationality of the results obtained from the theoretical study was verified. Through the acoustic signal acquisition system, the acoustic data in laser welding process were acquired, and the amplitude spectrum of sound field was obtained by the fast FFT transform. The results show that the experimental results are in good agreement with the theoretical values, and the proposed acoustic mechanism of keyhole is suitable for application.