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Rubber is the key component of a compression packer, which directly affects the success or failure of seals. Presently, the contact stress between the rubber and borehole walls is used as the main evaluation criterion for the sealing ability of the rubber of the compression packer. However, this evaluation criterion is imperfect. This study establishes the sealing performance and reliability evaluation system of a compression packer at 120°C. Considering the influence of time and temperature on the mechanical characteristics of rubber materials, the international advanced test technology was used to complete the uniaxial, planar and biaxial tensile tests and stress relaxation test for hydrogenated nitrile butadiene rubber (HNBR) of a compression packer under high temperature. Next, the hyperelastic and viscoelastic constitutive models of rubber materials based on high-temperature test data were established. Comprehensive sealing performance and reliability evaluation system for the compression packer rubber with compression rate, linear pressure, surface pressure and sealing performance coefficient were proposed. The evaluation system tested the rubber's stress relaxation effect on the rubber's sealing performance. The establishment of the evaluation system improved the shortcomings of the current packer-sealing performance evaluation system and provided a theoretical basis for the design and optimisation of the key structure of the packer-sealing system. Besides, based on the evaluation system, a platform for automatic structure simulation analysis and optimisation design of packer sealing system was developed and successfully applied to the actual design and optimisation process of packer. The system can effectively predict the sealing ability of the packer and guide field applications of the packer.