Abstract: In ultra-narrow gap welding, high quality welded joint could be obtained by setting matching process parameters according to its groove width, but the arc instability occasionally occurred. This phenomenon was mainly due to the influence of random interference factors on groove width, such as the flatness of groove sidewall. Under conditions of fixed and variable groove width, the influence of external characteristics of power source on the arc stability was studied and the welding results were analyzed. The results showed that the matching descent characteristics could enhance the arc stability when the groove was slightly deformed.