Hydrogenated nitrile rubber was one of the commonly used materials for downhole tools during drilling and completion, and its performance directly affected the safe operation of downhole tools. After hot rolling of hydrogenated nitrile rubber under different fluid conditions, the mechanical properties and volume changes of the rubber were tested through tensile performance experiments, tear strength experiments, compression permanent deformation experiments, etc. Combined with infrared spectroscopy and tensile fracture characteristics, the performance and structural changes of hydrogenated nitrile rubber in the underground environment were analyzed to clarify the performance degradation mechanism of hydrogenated nitrile rubber in the underground environment. The results showed that the tensile strength and tear strength of hydrogenated nitrile rubber were significantly reduced under different fluid conditions. In terms of volume, thermal rolling in water-based drilling fluid caused an increase in compression deformation, but the rate of volume change and mass change decreased; Oil based drilling fluid could cause permanent deformation of rubber compression, increased in volume change rate, and increased in mass change rate. The reason for the decrease in strength and volume in water-based drilling fluid was due to the oxidation of functional groups and the invasion of salts, which caused molecular curling; The reason for the decrease in strength and increase in volume of oil-based drilling fluid was due to the oxidation of functional groups, oil infiltration, and the dissolution of additives. The research results provided technical support for the optimization of hydrogenated nitrile rubber and downhole tools.