The invention relates to the field of welding and in particular relates to an ultrasonic vibration assisted welding method. The method comprises the following steps: in a welding process of fusion welding, providing an ultrasonic vibration force through an ultrasonic excitation vibration device; introducing ultrasonic frequency stress wave into a welding fusion tank; finally, transmitting vibration to a welding part. With the adoption of the configuration, the quantity of crystal nucleuses can be effectively increased, the size of crystal grains is prevented from growing, dendritic crystals are crushed and a crystal shape is changed, so that condensed welding line metal crystal grains are thinned; gas, impurities and the like in the fusion tank are prompted to rapidly float up, and defects of air pores and slag adding are reduced; welding residual stress and welding deformation are reduced; components are more uniform so that a welding line metal structure is improved; a fusion region becomes narrow and is uniformly transited, and non-fusion defects are reduced; comprehensive mechanical properties of welding lines are improved through the effects above.