Through the analysis of the displacement faults of the top seal and the spindle of a steam turbine in a chemical plant,it is found that the wear of thrust bearings will be accelerated when the air leakage increases to a certain extent and the shaft displacement of the steam turbine will gradually increase,which indicates that the damage of the seal not only causes the increase of leakage,but also causes the increase of the spindle displacement.The flow field of the top seal was simulated by using the FLUENT software,the influence of radial and axial clearance of the top seal on the leakage and the displacement of the main shaft of the turbine was studied,and the reasonable working clearance was put forward.The results show that the radial clearance of the steam seal on the top of the blade is an important factor affecting the leakage,and the increase of the radial clearance will lead to the increase of the steam leakage and the increase of the axial thrust of the steam turbine,which will lead to the increase of the spindle displacement.When the total length is fixed,the axial clearance has no obvious effect on the steam leakage,however the increase of the axial clearance can make the steam pressure to the seal more stable.Considering the assembly and thermal expansion of the blade top steam seal,a repair scheme with radial clearance of 0.65 mm and axial clearance of 3.5 mm was put forward for the studied blade top steam seal.Practice proves that the repair scheme is successful in site repair.