The pretreatment of renewable lignocellulosic biomass is essential for the production of fermentable sugars and biofuels in a sustainable biorefinery. However, it has been reported that lignin repolymerization during pretreatment enhances lignin inhibition in the enzymatic hydrolysis of cellulose. The search for green additives to weaken lignin inhibition is of great importance for a sustainable biorefinery with lignocellulose. In this work, green additives, mannitol and syringe acid, were thoroughly investigated in order to improve the effectiveness of the pretreatment of hardwood with dilute acid. The results showed that these additives, when pretreated with dilute acid, changed the structure and physiochemical properties of lignin, thereby reducing lignin inhibition in cellulose hydrolysis. In detail, 31P-NMR results apparently showed higher aliphatic OH groups of lignin after addition of mannite. 2D HSQC-NMR analysis showed more β-O-4 and fewer β-β structures, while GPC analysis showed lower molecular weight of lignin. The results indicated that mannitol was incorporated into the lignin structure to form etherified lignin with many hydroxyl tails, which suppressed the repolymerization of lignin. As a result, the surface barrier effect of lignin was largely reduced by the addition of mannitz, which led to an ease of cellulose hydrolysis which was improved by up to 53%. To the best of our knowledge, mannitol as an additive in the pretreatment of biomass has not been reported. In this work a novel green additive for the suppression of lignin repolymerization was presented in order to promote the enzymatic saccharification of cellulose in a sustainable lignocellulose biorefinery.