Saccharina japonica was fractionated by dilute sulfuric acid to increase the glucan content. The optimal fractionation conditions were determined as follows: reaction temperature 141.14 ℃, reaction time 27.85 min and catalyst concentration 0.30%. The CCD model predicted 32.83% glucan content under these conditions. Experiments confirmed the maximum glucan content of 32.67% under the optimal reaction conditions, which was 4.7-fold higher than that of the raw S. japonica (6.95%). With the residual solid, an enzymatic digestibility of 89.38% was obtained using 15 FPU/g-glucan of cellulase enzyme loading, which was 2.6-fold higher than that of the raw S. japonica (34.85%).