The study investigated the chemical composition, supramolecular structures, particle size distribution, and enzymatic efficiency of Phragmites australis lignocelluloses (PALs) with different moisture contents and treatment with 5 MeV electron beam irradiation. After irradiation, regardless of the moisture content, all PAL samples were degraded, their supramolecular structures were damaged, and the proportion of small PAL particles and the PAL enzymatic conversion rate increased significantly. There was no obvious difference in the apparent morphology, chemical structure, or crushed particle size distribution of PAL samples with different moisture contents when the absorbed dose was the same, but the enzymatic efficiency was negatively correlated with moisture content. The cellulose enzymatic conversion rates of PAL samples with 5%, 10%, and 50% moisture content were 22.24%, 19.76%, and 18.57%, respectively, and the enzymatic conversion rates of hemicellulose were 25.04%, 23.84%, and 19.56%, respectively, after 500 kGy electron beam irradiation. Meanwhile, the cellulose enzymatic conversion rates of PAL with 5%, 10%, and 50% moisture content were 54.09%, 47.27%, and 49.24%, respectively, and the enzymatic conversion rates of hemicellulose were 62.30%, 53.25%, and 47.83%, respectively, after 1 000 kGy electron beam irradiation. When the absorbed dose was 500 kGy, PAL cellulose and hemicellulose samples with 5% moisture content were severely degraded compared to samples with 10% and 50%. Meanwhile, when the absorbed dose was 1 000 kGy, the PAL cellulose and hemicellulose samples with 50% moisture content were severely degraded compared to those with 5% and 10% moisture content. Moreover, more cellulose and hemicellulose with 50% moisture content in PAL were degraded to non-carbohydrate substances compared to those with 5% and 10% moisture content.