Abstract: A method for obtaining a reinforced packaging paper by applying a residual fluid from the pre-treatment of lignocellulose waste with a water-alcohol mix to the packaging paper and the subsequent drying thereof. The residual fluid has a solid fraction in suspension of 1-70% by weight/volume relative to the total, with lignin and lignin-carbohydrate complexes; and a liquid fraction of 30-99% by weight/volume relative to the total, containing between 0.04 and 2.07 g/L glucose, between 0.31 and 14.57 g/L xylose, between 0.09 and 3.46 g/L arabinose, between 0.083 and 12.40 g/L acetic acid, and between 0.004 and 8.73 g/L furfural. The method improves the efficiency of the cellulose pulp residual fluids in the process of reinforcing a packaging paper, and is suitable for all types of raw cardboard or paper.