Hot water extraction of poplar wood was conducted at temperatures from 190 to 200 °C for 5 to 8 min. A hemicellulose yield of 81% and a lignin yield of 38% were obtained at 200 °C for 8 min. A combined process of microfiltration, ultrafiltration, and activated carbon adsorption was developed to separate lignin and sugars in the hydrolysate of hot water extraction. Lignin recovery efficiencies of 56.7%, 26.0%, and 13.2% were attained for microfiltration, ultrafiltration, and activated carbon adsorption, respectively. The characterization of lignin revealed diversity in molecular weight and functional groups, which is beneficial for high-value valorization. The obtained hemicellulose sugars from the combined process showed a good recovery rate of 43.8% and remarkable purity of 97.5%. The purified sugars were a mixture of monomers and oligomers that consisted of arabinose, galactose, xylose, glucose, and mannose. Sugar oligomers with degrees of polymerization from 2 to 6 accounted for 21.6% of all sugars.