The propagation characteristic of ultrasonic in repaired components of BT20 titanium alloy and molten pool is investigated by using ANSYS finite element simulation and experimental test. Under the circumstance that the horn vibrates longitudinally, the results show that when the length of repaired component coupled with horn is less than 23% of the longitudinal wavelength, the type of ultrasonic vibration on the surface of repaired component belongs to longitudinal vibration; however, when the length of repaired component is more than 28% of longitudinal wavelength, the type of ultrasonic vibration belongs to bending vibration; With the length of repaired component increasing, resonant frequency changes a lot and redistributes when the length of repaired component is between 23% and 28% of the longitudinal wavelength. In the two different kinds of vibrations, as the length of repaired component increases, the resonant frequency declines. When ultrasonic vibration is loaded at a frequency of 19.56 kHz, with amplitude of 0.2 μm, the positive and negative acoustic pressure in the molten pool changes alternatively and the negative pressure reaches hundreds of thousands of pascals.