In order to improve the laser welding quality of TC4 titanium alloy, a technology named ultrasonic processing across different phases based on laser welding (UPPLW) is introduced. With the three-circle-cutoff-point method, the effect of ultrasonic vibration field power on the microstructures of laser welding joints is studied by changing the intensity of ultrasonic vibration field, and the measured data are analyzed based on the reliability theory. The results show that, under the influence of the ultrasonic vibration field, with the increase of ultrasonic vibration power, the microstructure grain of the welding joints has a tendency to be progressively refined. When the power is 1600 W, the grain is refined by about 15.8% compared with that obtained with conventional methods. The mechanism for UPPLW to refine grain is finally analyzed.