Introduction   During the actual operation of the power plant， due to the inaccurate calculation of the effective height of the water seal， the water seal was pulled or worn， or the water seal was not well drained， thus affected the economic and safe operation of the unit.

  Method  In order to provide a more accurate calculation of the effective height of the water seal in the subsequent works， the influencing factors were analyzed and discussed which were the position of the water seal， the structural size of the water seal， the back pressure change of the condenser， the pressure change of the shaft seal fan and the hydrophobic vaporization.

  Result  The influencing factors of the above factors on the calculation of the effective height of the water seal are obtained. When calculating the effective length of the U-shaped water seal， the influence of the above influencing factors needs to be comprehensively considered， the initial calculation value is corrected， and the height of the multi-level water seal is reasonably selected.

  Conclusion  So as to solve the problems occurring at the scene，and the guiding opinions and precautions for design， construction and commissioning are proposed.