The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Nio.8 Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer.It was found that the sweep rate of 13.6kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.