Abstract: In order to study the droplet transfer controlling associated with CO₂ welding quality, the method of quadratic timefrequency analysis was used to investigate the signal of arc voltage to obtain the patterns of time-frequency distribution.The further characteristic information, namely energy spectral density and instantaneous frequency estimation of the signal, was obtained on this basis of extraction to the information of time-frequency image during welding process.Experimental results showed that the time-frequency energy distribution of arc welding signal can be studied by means of Choi-Williams distribution analysis.The characteristic information of time and instantaneous frequency in arc physical phenomena such as short circuit and arc broken can be extracted according to the results of time-frequency analysis.There is a different characteristic about instantaneous frequency at singular point, which can be used to recognize the physical phenomenon of the arc.