The gas–liquid cylindrical cyclone (GLCC) employs gravitational and centrifugal forces to realize gas-liquid separation. The aim of this study is to understand the droplet size distribution and pressure control in the GLCC via experiment and numerical analysis. The droplet size and pressure distributions were measured using Malvern RTsizer and pressure transmitters, respectively. The Discrete Phase Model was used to numerically analyze the swirling hydrodynamics of the GLCC. The results showed that the increase in the gas superficial velocity decreased the droplet size distribution at the inlet as a whole due to the shear effect and flow instability. The increase in the liquid superficial velocity only increased the small droplet size distribution at the inlet for the limitation of the gas’s carrying capacity. The pressure loss mainly occurred at the inlet and the overflow outlet. When the liquid level was remained below the inlet and above the liquid outlet, the liquid level and the liquid outlet section approximately met the Bernoulli equation for a finite large flow beam. With the increase in the pressure at the gas outlet, the liquid film fell back and the separation efficiency increased gradually. These results are helpful for further spreading applications of the GLCC in industry.