Under simulated conditions hot hydraulic liquid accelerated aging testing of nitrile-butadiene rubber seal was carried out. By the FT – IR,DSC,TGA and scanning electron microscopy methods; the variation of chemical structure, thermal property and fracture morphography of rubber seal was analyzed in hydraulic fluids. The results show that after aging testing, the compression property of the rubber seal decreased gredually, both thermal decomposition temperature and glass transition temperatures increase. On the one hand, under the effect of stress, molecular chain segments have been orientated in some directions, restricting the relaxation of molecular chains, which resulted in the compression property decreased and the thermal property increased. On the other hand, the tensile fracture surface tends to be smooth and have holes, and the content of additive decreases, which means embrittlement has occurred to rubber seal and there have been mass exchange between rubber seal and hydraulic fluid, this is the other factors of compression property decreased.