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In order to improve the laser welding quality of TC4 titanium alloy, an technology named ultrasonic processing across different phases based on laser welding was introduced to assist laser welding, and the effects of ultrasonic vibration field on the macromorphologies, microstructures and microhardness of the laser welding joints were studied by changing the ultrasonic vibration power. The results show that under the effect of the ultrasonic vibration field, with the increase of ultrasonic vibration power, there are clumped concave appear on the welding surface, and the surface roughness of the welding joints is lower, meanwhile, the acicular α′-martensite content is more intense, orientation and arrangement are more cluttered, besides, the acicular α′-martensite average length is shorter because of ultrasonic vibration and radiation pressure. The microhardness of welding joints increases, but the microhardness of heat affected zone and the base metal area remain unchanged with the increase of ultrasonic vibration power.