The Static softening behavior in the two-stage isothermal compression of AZ31B magnesium alloy is investigated at the deformation temperatures ranging from 250℃ to 450℃, strain rates ranging from 0.005s-1 to 1s-1 and the inter-pass times ranging from 15s to 240s after the first stage isothermal compression. The prediction model of softening rate was established. According to the essence of rolling residual strain rate, the calculation method of residual strain rate in multi pass rolling process was proposed. The results show that the static softening fraction increases with the increasing of deformation temperature, strain rate and the inter-pass times. The softening rate calculation model can be used to characterize the softening rule of AZ31B magnesium alloy, the average relative error is 12.58%.The calculation model of residual strain rate can provide technical support for solving the deformation resistance of AZ31B magnesium alloy during multi pass deformation process.