In this study, galactomannans, which are a type of plant polysaccharide, were oxidized by using a laccase and TEMPO-mediated air-oxidation system. The influence of the oxidation conditions, including the laccase doses, oxidation times, and amounts of oxidizing system, on the properties of the paper were investigated. The results showed that the oxidized galactomannans could improve the paper properties. The optimal conditions were as follows: a 60 U/g enzyme dosage; 6-h oxidization time and a 0.1 g TEMPO dosage. When compared with the control paper, the tensile index and folding endurance have been improved by 28.42% and 88.02%, respectively, when 1.5% oxidized galactomannans were used, and for the recycled paper, the increment could reach 126.97% and 43.85% correspondingly. Also, the results of the elemental, FT-IR, and CP/MAS13C NMR analyses demonstrated that some of the hydroxyl of the galactomannans had been transformed into the carboxyl and aldehyde groups.