In the cold metal transfer plus pulse (CMT + P) welding process of Ti-6Al-4V, a tungsten needle with ultrasonic vibration was inserted into the front side of weld pool to stir the molten pool, thereby refining the grains and improving the tensile properties of weld joint. The experimental results showed that under the experimental conditions, the ultrasonic vibration of molten pool can effectively inhibit the columnar crystal growth in the weld zone and improve the tensile properties of weld joint. When the tungsten needle was placed closer to the rear of molten pool, the effect of the ultrasonic vibration of molten pool was more significant. The ultrasonic vibration of molten pool did not change the phase composition of weld zone, but it affected its grain orientation and relative content. All tensile specimens were fractured at the weld zone, and the fracture mode was ductile / brittle mixed fracture.