The primary wind of a low-NOx coaxial swirling burner was visualized by using glycol as smog tracer. The information of the visual flow field was input into a computer through image-capturing card with CCD camera as the image-capturing element. The boundary of the visual zone, i.e., the interface of the primary wind and secondary wind was obtained by image processing. The fractal dimension (FD) of the boundary was examined and found to vary from 1.10 to 1.40 with S1, S2 and ζ1. It is concluded that when FD is small, the complex level of the interface is low, and mixture between the primary and secondary wind is weak near the exit of the burner at the initial phase of combustion resulting in stratified flow; when FD is big, mixture becomes strong near the exit of the burner. It is showed that the flow with FD ranging from 1.10 to 1.20 is stratified flow, which is benefical to reduce NOx yield and the flow with FD from 1.25 to 1.40 is mixed flow, producing much NOx. The mechanism of the forming of stratified flow and mixed flow was theoretically analyzed. The corresponding S1, S2 and ζ1 of these flows were given.