<p>Solid mechanochemistry, as an environmentally sustainable technology, is widely applied to prepare functional materials, but suffers from some issues such as limited reaction from short exposure time and low reactivity within the solid phase. Here, we proposed a combined approach of mechanochemistry and surface-confined reaction to facilitate cellulose phosphorylation. Specially, cellulose with 4 wt% H₂O are mixed together to form a hydration layer on the surface followed by a conventional ball milling treatment. The confined-water layers, as a reaction medium, could not only cause location ionization of hydroxy groups in cellulose to reduce the activation barrier, but also immobilize phosphorylating agents on the cellulose surface, facilitating the phosphorylation process. As a result, the phosphorylated cellulose demonstrated a high degree of substitution of approximately 0.087. To this end, the potential of the phosphorylated cellulose as a promising flame-retardant, was demonstrated in wood pulp paper and polyvinyl alcohol film. This study highlights the promoting effect of confined water on the mechanochemical phosphorylation of cellulose, and can also be extended to preparation of functional cellulose.</p>