Many research efforts were performed to find the seepage effects on turbulent open-channel flow, from which results were often inconclusive and sometimes contradictory. A series of experimental studies are carried out to examine how seepage effects the open-channel flow structure and the velocity field is measured using the PIV image acquisition system in this study. The relationship between the flow velocity variation and the relative seepage intensity is analyzed. The results show that the velocity of water flow is not a monotonic function of relative seepage intensity. With the increase of injection intensity, the near bed current velocity decreases firstly, following by slight increase, and then decreases. With regards to the suction, the results are opposite. Measured data show an inflection value in the case of suction or injection, with the corresponding relative seepage intensity related to the Reynolds number. The larger the Reynolds number is, the smaller the relative seepage intensity occurs when the inflection value appears.