The conversion of cellulose from renewable biomass into the key platform chemical, 5-hydroxymethylfurfural (HMF), is of fundamental importance to the production of numerous bioproducts and biofuels. Various acidic catalysts have been developed for this process; however, most systems suffer from low efficiency and poor sustainability. Here, we report an acid-free conversion of cellulose into HMF in a biphasic system of tetrahydrofuran/concentrated seawater. The yield of HMF reached 48.6%, and this system has excellent reusability and sustainability. We found that the chloridions (Cl-) can promote the isomerization of glucose via a 1,2-hydride shift path and accelerate the dehydration of fructose, thus driving the selective formation of HMF. This simple system is capable of converting raw biomass to furfural and HMF with the lignin residues transformed into useful alkanes via a sequential catalytic upgrading, paving a new economic-viable pathway for the full valorization of lignocellulosic biomass.