The freshwater shortage has become one of the most urgent and serious problems for the sustainable development of human society due to the limited quantity and huge demand. Thus, various kinds of technologies and corresponding materials have been studied and evaluated to either expand the freshwater source or enhance the utilization rate in the water cycle. Cellulose, as the most abundant biopolymer in nature, shows some unique characteristics like low cost, non-toxicity, and biodegradability, making it popular as a supporting material in water treatment. Integrated with different kinds of functional materials, the composites could be applied in relevant technologies for wastewater treatment and freshwater generation. In this thesis, cellulose-based composites were designed, fabricated, and applied in dye degradation, bacterial killing, and interfacial solar evaporation. For the dye degradation and bacterial killing, Copper-Benzenedicarboxylate (CuBDC) was grown on the skeleton of cellulose aerogels with a lightweight and porous structure. The composite aerogels showed effective catalytic degradation towards common dyes and good antibacterial performance towards various kinds of strains. As for interfacial solar evaporation, polypyrrole (PPy) was introduced on the wood-inspired cellulose aerogels with vertical channels by in-situ polymerization. The fabricated composite aerogels exhibited excellent evaporation performance under one sun illumination and good stability towards saline water due to efficient solar-thermal conversion, fast water transport, decreased enthalpy for water vaporization, and quick migration of salt ions. To further investigate the feasibility of interfacial solar evaporation applied in wastewater treatment, the fabricated PPy/cellulose aerogels were introduced to treat industrial wastewater from hydraulic fracturing operations with high salinity and complicated composition. The condensed water was collected by a purpose-made collector and showed dramatically improved quality compared with original wastewater. To design and fabricate interfacial solar evaporators with more complicated structures, carbon nanotubes (CNTs) were mixed with cellulose as the composite ink for 3D printing. The printed solar evaporator showed good performance and the impact of various parameters on evaporation performance was investigated. Research work in this dissertation indicates the great potential of cellulose-based materials applied in water purification and freshwater generation by interfacial solar evaporation, further prompting their applications in practice.