Delayed detached eddy simulation (DDES) is accompanied with Stereo-PIV measurements to study the non-reacting flow field of a non-premixed swirl burner in this paper. Comparisons of experimental and numerical data show that the DDES results are capable of predicting the mean swirling flow features adequately. The instantaneous flow field is found to be strongly affected by the Kelvin–Helmholtz instability. The flow near the injector involves a complex behavior including a recirculation zone. The 3D flow structure at the burner exit, visualized by the iso-surface of Q-criterion, displays four instability types. The dominant instabilities are vortex ring structures induced by the Kelvin–Helmholtz instability, and finger structures induced by the swirling instability. Pressure fluctuation signal recorded in the swirling jet region show that the computational flow passes through transition instants from RANS to DDES equations. After that, the swirling jet becomes fully developed with an oscillation frequency of 222 Hz.