A novel gas-liquid cyclone characterized by the generation of swirl flow via guide vanes and a uniflow stream was designed for the pre-separation of a horizontal gravity based separator. The internal flow field and separation performance of the cyclone were investigated by numerical simulation. The Eulerian-Lagrangian approach with the Reynolds stress model (RSM) was used in the simulations. Contours of velocity and pressure within the cyclone are shown, and the trajectories of the droplets are also presented, demonstrating the separation mechanism of the cyclone. Then numerous simulations were conducted with different structural parameters to optimize the cyclone performance. The results show that broadening the width of the gap lw is good for large droplets separation; Decreasing the discharge angle a or increasing the torsion angle beta of the guide vanes can increase the tangential velocity and improve the separation efficiency, but the pressure drop also increases fast; The increase of the diameter of the central body Dc will lead to an increase of tangential and axial velocity, and the pressure drop increases significantly as well.