To explore the feasibility of the online turbidity meter installed in-line, we have to study the influence of flow velocity on in-pipe turbidity detection. In the laboratory self-designed water circulation system, the correlation between the in-pipe turbidity and flow velocity was studied by altering the water flow rate in the pipe. The results show that the in-pipe turbidity and flow velocity have a benign linear relationship for the slow flow; as the flow velocity increases, the in-pipe turbidity increases slightly but is limited. However, there is no significant correlation between the in-pipe turbidity and flow velocity for the fast flow.