In the present work, we experimentally investigate the role of the unsteady swirl flow field on the internal two-phase flow evolution of a simplified coaxial piloted pre-filming injector with required optical access. A suite of diagnostic techniques, including direct laser sheet imaging, time-resolved stereo particle image velocimetry (TR-SPIV), time-resolved volumetric laser induced fluorescence (TR-VLIF) imaging, time-resolved high resolution planar LIF imaging, and phase Doppler interferometer (PDI) measurements are implemented to capture the internal two-phase flow field of the injector. As the primary swirl flow operates in the strong swirl regime, a precessing vortex core (PVC) is generated and induces a precessing motion to the pilot nozzle spray. The spray precession is quantified by TR-SPIV. The consequent periodic impingement of droplets on the prefilmer is shown by TR-VLIF to result in a non-uniform wall filming. The variation of the liquid film thickness is captured by the high resolution PLIF imaging. The film accumulates at the tip of the prefilmer and forms a thick liquid rim, which is sheared by the primary and secondary swirl air into ligaments and large droplets. Besides, fine droplets directly from the pilot nozzle and also originating from the splashing of droplets on the liquid film exit the prefilmer. Spectral analysis shows that the frequencies of primary spray precession, film thickness, and fine droplets exiting the prefilmer match closely, corresponding to the primary swirl PVC. By contrast, the breakup of the liquid rim into ligaments is quasi-periodic, so the liquid accumulation at the prefilmer tip smears the effect of the PVC on the external spray atomized from these ligaments. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.