Abstract: The water and sediment separation performance of the Vortex Settling Basin (VSB) depends on the characteristics of the velocity field. To understand the influence of the opening of the deflector on the flow field, VSBs with the deflector partially perforated on the non-overflow area entirely perforated and without any openings were tested with Particle Image Velocimetry (PIV) and simulated with a large eddy turbulence model. The results showed that:① The opening positions did not affect the vortex properties of the VSBs but did affect their strength. The swirl intensity of the entirely perforated VSB was the smallest, which made it difficult to extract sediment by centrifugal force. ② The VSBs without any openings were partially perorated helped the sediment to be transported along the radial direction, move down and be discharged, but the radial velocity and secondary flow in the entirely perforated VSB were reduced, which caused the sediment to be deposited on the floor. ③ The velocity on the partially perforated deflector was the greatest, which could prevent the sediment from depositing on the deflector. Therefore, to reduce the amount of particles deposited on the deflector of the VSB and to ensure a high sediment trapping efficiency, it is better to choose a deflector that is partially perforated on the non-overflow area.