Abstract: The mechanical seal is the key component of the reactor coolant pump in nuclear power plant. During the operation of a primary coolant pump,the pressure before the third-stage seal decreased and the leakage increased. A two-dimensional axisymmetric finite element model of the third-stage secondary seal for hydrodynamic mechanical seals of the reactor coolant pump was established based on a finite element software. The deformation behavior and contact mechanism was analyzed at the state of the installation stage,normal pressure and full system pressure. The influence of the sealed pressure on deformation,Mises stress distribution,contact width and contact pressure were studied. The results show that the secondary seal plays a sealing role under normal working conditions;the secondary seal contacts with the insert part of the seal,which causes the fretting of the metal insert part;there is a gap between the retaining ring and the insert part,so there is no possibility of wear between the retaining ring and the insert part. The displacement,Mises stress,contact pressure and contact width of the secondary seal increase with the increasing sealed pressure. The retaining ring prevents the extrusion of the flexible secondary seal and damage.