The impact of fibrosis degree of bleached kraft pulp (BKP) pretreated by cellulase on high-yield pulp (HYP) properties were studied in our laboratory. The results showed that the refining energy consumption decreased greatly by pretreating BKP with cellulase prior to refining. When 20%(wt) cellulase-treated BKP (40 degrees SR) was added into high yield pulp, tensile index, Z direction strength and tear strength of HYP handsheet were increased by 162%, 12% and 19%, respectively, compared with untreated BKP(40 degrees SR). Cellulase-treated BKP was more effective on HYP apparent density than untreated BKP, but opacity of HYP didn't change significantly when added BKP was pretreated by cellulase. The possible reason is that cellulase pretreatment improved BKP fibrosis degree, increased inter-fiber bonding area, and finally raised the strength properties of HYP.