Hydrogen peroxide presoaking prior to ammonia fiber expansion (H-AFEX) was applied to enhance fermentable sugars production from corn leaf. Effects of temperature and H2O2 loading on solid recovery, chemical constitution, and polysaccharide conversion were investigated. Physiochemical characterizations of raw material and H-AFEX-treated substrates were obtained using digital microscopy, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results showed that the H-AFEX process was an effective pretreatment for improving enzymatic hydrolysis of corn leaf. The HAFEX-treated substrates under optimal conditions following enzymatic hydrolysis achieved 91.6% glucan conversion, 80.6% xylan conversion, and a total monosaccharide yield of 411.8 g per 1000 g dry biomass. Further studies showed that the H-AFEX process induced morphological modification of corn leaf by removing minerals/extractives, relocating melted lignin, and generating pores. XRD data revealed that the crystallinity index of H-AFEX-treated substrate decreased. The disruption of crystalline structure, the removal of amorphous substances, and the transition of crystalline region to amorphous region were observed. FTIR data demonstrated that H-AFEX pretreatment disrupted hydrogen bonds inter-/intra-cellulose chains, altered the supramolecular structure of cellulose, and caused destruction to the molecular structure of residual lignin. As a result, the H-AFEX process significantly reduced the recalcitrance of corn leaf and facilitated biomass conversion to fermentable sugars.