This paper describes the visualization of water flow in a circular pipe with an axi-symmetric cavity under high Reynolds number by means of laser-light-sheet method Experiments have been carried out for various ratios of cavity length to cavity diameter. Experimental results show that a single steady vortex is produced in the cavity in the case that the length-to-diameter ratio is one, but the flow in the cavity becomes to complicated and the shear layer becomes to unstable as the ratio increase. The fluctuation of velocity and pressure generated in the pipe flow can be considered by the results obtained