Abstract: Against the demanding high-temperature and high-pressure (HTHP) conditions in ultra-deep wells for shear ram blowout preventer (BOP), this study investigates the high-temperature sealing mechanism and strategies for enhancing the seal integrity of shear ram. Utilizing a combined approach of theoretical analysis, finite element simulation, and experimental validation on an FZ35-105 BOP test, the research examines the influence of hydrogenated nitrile rubber (HNBR) material properties, the structural design of the shear blade seal and side seals, and the ram block configuration on high-temperature sealing performance. Results demonstrate that HNBR is the optimal sealing material for drilling well control equipment in HTHP environments due to its superior thermal stability and chemical resistance. Under extreme conditions of 177 ℃ and 105 MPa, an end-face sealing shear structure maintained effective sealing for 493 minutes. This represents a 17.1% improvement over the 421 minutes achieved by traditional contact sealing and far exceeds the standard requirement of 60 minutes. When the single-side compression of the side seal reaches 5.3 mm, the contact stress can achieve 105 MPa, thereby achieving zero leakage under wellbore pressure. An interlocking shear ram design reduced the clearance between the upper and lower rams by 40%~50%, effectively mitigating seal extrusion. A quantitative model correlating side seal compression to sealing pressure was established, with predictions deviating from experimental data by less than 10%. The study recommends prioritizing end-face sealing shear structures for large-bore BOP. These findings provide critical technical support for the design and manufacture of shear ram BOP capable of operating in the HTHP environments encountered in ultra-deep drilling, significantly enhancing their sealing performance and high-temperature resistance.