Abstract: This paper developed an experimental system based on arc light sensing.The molten pool is excited into oscillation by fluctuant welding current with linear increasing frequency in both stationary and travelling TIG welding of thin steel plate.When the frequency of external exciting current is equal to the natural frequency of molten pool,molten pool resonance happens.The amplitude of oscillating signal is acquired by arc light sensor.The feature signal of molten pool oscillation is then detected,which is identified as resonance signal and can be used to estimate molten pool size in real time.This research approach paves a new way for solution of penetration control in travelling TIG welding.