In order to accurately study the sealing characteristics of O-ring used for high pressure plunger pairs, considered the influence of fluid penetration into the seal contact area on contact pressure, a numerical simulation method for the comprehensive permeability and noncontact boundary was proposed based on the pointbypoint comparison method, and the contact pressure and von Mises stress distribution of O-ring assembly under high pressure were investigated. The influence of O-ring assembly structure parameters on the seal performance under different precompression and fluid pressure was discussed.The results show that the analysis method synthesized permeable boundary and noncontact boundary is highly reliable, and the test comparison error is 2.9%~4.3%; with the increase of O-ring precompression, the main contact pressure and sealing length increase significantly, but the internal von Mises stress and lateral contact pressure change little; with the increase of fluid pressure, the O-ring von Mises stress and primary and lateral contact pressures increase significantly, but the seal length changes little.The combined seal retainer applied to the high pressure plunger pair can effectively avoid the local stress concentration of the O-ring. The contact pressure of the retainer increases with the increase of fluid pressure, but it is significantly less than the main contact pressure of the O-ring, and only plays the role of auxiliary sealing.The research results provide a basis for the design and optimization of dynamic seal structure of high pressure plunger pairs.