Abstract: Reason analysis on two-time pipe fractures in hydrotest during drainage for a great fall pipeline was made based on four similar characteristics in the fractures. This pipeline was laid along an elevated area, and air was accumulated at the high spot of pipeline when water drainage was conducted in hydrotest. In the later stage of drainage, the backpressure of pressure head was quite high due to the small size of drainage hole, the air was compressed to burst under the push of pig, and a bridge water hammer was formed to produce an instantaneous high pressure to break up the pipe. According to estimation results derived from the water hammer boost pressure formula, the instantaneous high pressure caused by the bridge water hammer could be up to 41.6 MPa, and this result greatly exceeded the ultimate load of the pipeline. To verify the above conclusion, the drainage outlet was expanded to reduce the impact of water hammer, and the on-line monitoring on the pressure near the fracture point was carried out in the third drainage of hydro test with a satisfactory result. And the typical water hammer pressure signal with the peak value as 1.75 MPa was detected in the end of drainage stage, which proved that the air did exist.