The hierarchical structure of wood cell walls resulting from complex arrangement and distribution of the heterogeneous components is considered to impact significant impediment to enzymatic hydrolysis of cellulose for biofuels. In this work, micronized wood with significant cell wall ultrastructural deconstruction were effectively produced from ring and puck milling within 12 min. In a subsequent enzymatic hydrolysis, micronized wood resulted in increase of cellulose hydrolysability by 4–14 folds over that of starting material. The underlying mechanism towards facilitating enzymatic hydrolysis was studied through delineating the ultrastructural changes and alternation of cellulose chemistry in micronized wood cell wall using SEM, TEM, CLSM, GPC, XRD, HPLC and Simon’s staining techniques. Electronic microscopy revealed distinct stages of wood cell wall deconstruction that was coincident with particle size reduction, including cell fracture and delamination, cell wall disintegration, and amorphization of cell wall fragments. Simons’ staining results also indicated increasing substrate accessibility and porosity of micronized wood, likely due to the ultrastructure alternation of cell walls. GPC and XRD revealed significant decrease of cellulose degree of polymerization (DP) and crystallinity. The correlation of these factors with cellulose hydrolysability was studied and further arranged in order through principal component analysis. The major positive factors affecting hydrolysability were surface accessibility and porosity, while cellulose crystallinity and DP were the major negative factors accompanied by particle size. The established weighed order of factors behind hydrolysability provides insights of lowering cell wall structural recalcitrance by mechanical manner.