Poplar wood sawdust was chemically modified and separated into fractions using a mixture of p-toluene sulfonic acid (p-TsOH) and formic acid under different conditions. The optimum conditions of poplar lignin separation were determined by single-factor experiment. The mixed acid lignin (MAL) and the solid residues were subjected to comprehensive structural characterization by Fourier-transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H-NMR), X-ray diffraction (XRD), and scanning electron microscope (SEM). At the mass ratio of 5:1 of p-TsOH and formic acid, temperature of 80 °C, acid concentration of 75%, and reaction time of 20 min, more than 80% of lignin was removed, and almost all of the cellulose was retained in the solid residue. The results indicated that the p-TsOH/formic acid achieved rapid and nearly-complete dissolution of wood lignin below the water boiling temperature by enhancing the cleavage of interunitary bonds in lignin (β-O-4′ bond) and the 4-hydroxy-3-methoxy cinnamic acid structure in the lignin.