Hydraulic descaling is used in hot rolling mills in order to remove scale and prevent surface defects. Because the impact pressure of the descaling jet is one important factor from the viewpoint of mechanical breaking and applying thermal shock to scale layers, the water jet structure and the droplet velocity should have large effects on scale breaking properties. However, the influence of the injection distance on the jet structure and the droplet velocity has not been clearly understood. In this work, the behavior of changes in the descaling jet structure and attenuation of the water droplet velocity along the injection distance were investigated experimentally. High pressure descaling nozzles with pressures up to 25 MPa were used, and the injection distance was varied in the range from 30 to 400 mm. The jet structure was observed with a high speed camera, and the water droplet velocity and diameter were measured with a phase Doppler analyzer. The results confirmed that the jet structure changes continuously through a process of continuous flow, break-up, water lumps, and water droplets. It was found that a continuous flow can be maintained for a long distance by using a low injection pressure and large flow rate, and the water droplet diameter also becomes larger, which reduces velocity attenuation. These deformation properties of the jet structure are related to the Weber number expressed by the relative velocity between a water droplet and the surrounding air. A smaller Weber number is effective for reducing velocity attenuation over a long injection distance.