The Particle Image Velocimetry(PIV)technique is applied to explore the flow structures behind a hollow bridge pier. Hydrodynamic characteristics of the flow field past the pier with a rectangular hollow slot are examined in both horizontal and vertical planes. The result shows that the motion of the wake flow at the height of the slot can be changed. The development and movement of large-scale vortex is restrained and the upward motion trajectory of the bottom flow behind the pier is cut off by the strong shear effect that is generated due to the interaction between the high-speed flow pass through the hole and the low-speed flow in the lateral and back region of the pier. Characteristics of periodicity and high intermittency exist in the instantaneous flow field behind the pier.