To investigate the variation regularity of the characteristic droplet diameters in the swirling flow field, experimental research and numerical simulations were conducted. In the experiments, the white oilwater and the hydraulic oilwater mixture in a 100-mm-diameter pipe swirling flow field were tested. A Malvern droplet size analyzer and an electrical resistance tomography were used to measure the droplet size distribution and local oil volume fraction, respectively. In the numerical simulations, the Eulerian multiphase model, the population balance model, and the RSM turbulent model were coupled to simulate the droplet behavior in the swirling fluid field. The results revealed bilinear relationship (corresponding to the developing section and swirling section) between the section average d32 and the turbulent energy dissipation rate in the logarithmic coordinate system. Subsequently, quadratic relationship between the section average d90 and the swirling number owing to the dual influence mechanism of the tangential momentum on droplet coalescence and stabilization was found. Finally, linear relationship was observed between the d5 and the swirling number in the swirling section. Furthermore, the results indicated that these characteristic droplet diameters were sensitive to the operating parameters of the entrance Reynolds number and entrance oil volume fraction, as well as the oil properties, including the density, viscosity, and surface tension coefficient. All these factors are important for understanding the droplet evolution in the swirling flow field and can provide guidance for the design and optimization of swirling vane type de-oiling separators.