Frequent damage to the top seal of the reversed tainter valve of the high water head ship lock has become the main problem affecting the operation reliability of the lock，and it is necessary to study the hydrodynamic characteristics of the top seal working state. The top seal 1:1 section test device is used for the top seal gap jet hydrodynamic test in the small opening of the reversed tainter valve. The development of gap jet cavitation，the hydrodynamic load distribution on the seal surface and the plusation pressure energy distribution rules are revealed. The results show that gap jet cavitation produces high frequency pressure pulsation of about 20 to 80 Hz on the lower surface of the top seal，which is easy to cause instability of the large deformed flexible seal，and is the cause of shock vibration in the initial stage of valve opening.