To enhance the sealing performance and reliability of pressure-retaining sampling devices for coalbed methane, a three-dimensional model of an offset spherical sealing structure was established, and a thermo-mechanical coupling simulation method was employed to analyze the stress distribution and deformation behavior of the sealing assembly under high-temperature and high-pressure conditions. On this basis, a ground experimental platform was constructed to conduct pressure-retention tests at ambient and elevated temperatures, verifying the simulation results. The analysis results show that the device achieves effective sealing at a pressure of 3 MPa, with the equivalent stress and deformation of the polytetrafluoroethylene (PTFE) valve seat remaining within safe limits. Within the temperature range of 20~80 °C, both stress and deformation increase proportionally with temperature, yet the sealing requirements are consistently satisfied. Verification results indicate that the device maintained stable pressure for 5 h under ambient conditions at 3.2 MPa, with pressure loss rates of 3.1% and 6.2% at 8 h and 15 h, respectively; no significant pressure decay occurred within 3 h at 80 °C. These findings demonstrates that the device can effectively operate under high-temperature, high-pressure, and complex sampling conditions in coal mines, significantly improving the accuracy and reliability of pressure-retaining sampling for coalbed methane, and providing technical support for the exploration and development of coalbed methane resources.