In order to understand the hydraulic characteristics of the water flow in open channels with submerged flexible vegetation, experiments were carried out using the particle image velocimetry (PIV) method. The synthetic aquatic vegetation was used in place of natural flexible vegetation, and the constant water flow velocity field of the open channel with this vegetation was measured under different vegetation density and flow conditions, form which the longitudinal velocity, Reynolds stress, turbulence intensity along the vertical distribution and the variation of Manning coefficient were obtained under the condition of submerged vegetation. The test results show that the distribution of the longitudinal velocity can be divided into three zones, which are the free water zone, the middle canopy zone and the near bed zone. The flow velocity in the free water zone increased with the vegetation density, whereas it decreased with the increase of the vegetation density in the middle canopy zone and the near bed zone. The peak values of Reynolds stress and turbulence intensity appeared around the top of the vegetation crown and increased with vegetation density.