The mulberry branch can be used as a renewable biomass resource for the production of biofuels since it contains a significant amount of lignocellulose. However, the rapid deactivation of enzymes during lignocellulose degradation in a high-temperature environment leads to high costs and low efficiency of enzymatic saccharification. In this study, xylanase was modified by rational design, and xylanase and cellulase were used to synergistically degrade mulberry branches pretreated with seawater. The fermentable sugar yield and degree of synergy (DS) were determined. The dominant mutant S21Y/N318W of Hortaea werneckii xylanase Hwxyl10A was obtained with improved thermostability (T₅₀ increased by 12 °C, t_1/2 is 17-fold that of WT). Meanwhile, the specific activity at 75 °C increased from 1470 to 1901 U/mg. Under the condition of seawater immersion pretreatment, the highest yield of fermentable sugar was 313.5 μmol/g, and the highest DS was 1.57 after enzymatic hydrolysis of mulberry branches by cellulase and S21Y/N318W at 50 °C. Therefore, the biotransformation of mulberry branches reducing sugar has been significantly improved, which provides an efficient method of biomass saccharification for biofuel production.