Abstract A three-dimensional transient numerical model of a pulsed GTA welding is established based on the volume of fluid (VOF) method to study the oscillation of stationary weld pool surface. The detecting and sensing method based laser vision is used to measure the oscillation frequency in the weld pool. In conjunction with the theory, the oscillation frequency during welding process is analyzed. The free surface oscillation was investigated when the current changes from peak to base and when the weld pool was fully penetrated. Before full penetration, the weld pool oscillation frequency decreases as the welding time increases. The penetration depth increases with the welding time. When the molten pool reaches full penetration, the oscillation frequency stays the same. With the state of weld pool changes from partial penetration to full penetration, the oscillation frequency decreases rapidly. When the current changes from peak to base or from base to peak, the damping vibration takes place in a higher frequency. When the pool diameter is greater than 8.4 mm, the base metal fully penetrated.