The current in sea can maintain the water quality in the cage, but the strong current can cause serious deformation of the cage and reduce the effective volume of the cage. This study was conducted to research the reduction in flow velocity downstream from square cages in currents. The flow-velocity distribution downstream from cages was invested with four different net mesh shapes and sizes. The reduction in flow velocity was recorded using both the Particle Image Velocimetry (PIV) and Acoustic Doppler Velocimetry (ADV). The results showed that there was an obvious reduction in flow velocity downstream in the four cage models. The height of the flow-reduction region was approximately the same as the height of the cage. The flow velocity increased at the bottom of the cage. The results achieved by PIV consistent with the data recorded by ADV. The average reduction factor of the four cage models was from 0.5 to 0.8 at 0.18 m/s the incoming velocity. The reduction in flow velocity of the cage with small meshes was greater than the cage with big meshes. The reduction in flow velocity of the cage with diamond meshes was slightly greater than the cage with square meshes. The average reduction factor was not obviously impacted by the incoming velocity. The results of the vertical plane through the center of the cage by PIV were visualized by the velocity nephogram. These findings will contribute to the understanding of the flow characteristics around the square cage.