Abstract: With considering the characteristics of keyhole plasma arc welding and using three-dimensional conical heat source model, a three-dimensional model of molten pool is established.Then, the mode modal analysis of molten pool for China Low Activation Martensitic(CLAM) steel is conducted within the frequency range of 15 kHz to 100 kHz.Finally, the six main resonance center-frequencies(14 modes) are obtained and the response corresponding to five modes are analyzed.The results show that with the increase of natural frequency the local vibration deformation of molten pool is more and more severe.We can use this method to determine the resonant frequency of ultrasound-arc welding.This paper also provides some reference for process parameters optimization of ultrasound-arc welding as well as shortens the RD cycle and reduces the cost.