Abstract: The selectivity of thermostable and alkaline tolerant xylanase X on the pretreatment and pine kraft pulp bleaching was studied according to the change of pulp viscosity and kappa number before and after the pretreatment. The effect of the pretreatment on D0E1D1E2D2 bleaching properties was revealed by the changes of brightness of each bleaching stage and final viscosity. The results showed that the pretreatment had high selectivity without causing an obvious decrease of the pulp viscosity. The effective pretreatment conditions can be varied among pH 8.0—9.5 and 75—80 ℃. The final brightness of the bleached pulps pretreated with xylanase X was 1 % ISO higher than that of no pretreated one (90 % ISO brightness), but the viscosity lowed little under the same bleaching conditions or the ClO2 dosage in the D0 stage could be saved by 26 %—27 % to achieve the same brightness (90 % ISO) of the nopretreatment bleached pulp.