In order to understand hydrodynamic characteristics of circular gravity cage in varied flow-field, and obtain both the flow-velocity variations and flow-field distribution inside and around cages, four models(C1,C2,C3,C4) were designed to be measured at specific measurement points at a velocity of 0.068m/s,0.105m/s,0.143m/s,0.182m/s by particle image velocimetry (PIV) technique and acoustic Doppler velocimeter(ADV).The experimental data showed that the measurement result of ADV was basically same with that of PIV. The ratio of flow-velocity reduction inside a cage remained approximately the same in different current. On the effect of the flow-velocity reduction ,the cages with small mesh did better than the cages with big mesh and the cage with diamond mesh did better than the cage with round mesh at the same velocity. As current flowed through multiple cages, the flow velocity was gradually reduced. The flow velocity inside and downstream from the cages decreased with increasing number of net cages. The flow-velocity reduction was 67.6% inside the second cage, 88.2% inside the forth cage,72.1%downstream from multiple cages. This research can provide a reference to a suitable type of net cages used in production practice.