To elucidate the mechanism by which deep eutectic solvent (DES) pretreatments improve enzymolysis efficiency, acidic (choline chloride-lactic acid (ChCl-Lac), betaine (B)-Lac), alkaline (ChCl-ethanolamine (ChCl-M), and ChCl-N-(2-hydroxyethyl) ethylenediamine (ChCl-N-2)) DESs were used to pretreat corn stover and the effects of acidic/basic DESs on corn stover composition and enzymolysis efficiency were compared. Results indicated that alkaline DES is more effective in improving enzymolysis efficiency, with increased polysaccharide content (from 51.59% (untreated) to 81.33%~83.36%), lignin removal rates of 71.35%~89.72%, and the near-complete degradation of all polysaccharides. Fourier transform infrared (FT-IR) analysis indicated that pretreatment cleaved many of the lignin-hemicellulose bonds and enhanced the absorption peak associated with glycosidic bonds. X-ray diffraction (XRD) analysis demonstrated an increase in the crystallinity index (CrI) of corn stover following pretreatment, from 32.99 to 53.60, with no associated changes in the crystal structure. Furthermore, corn stover that is pretreated with alkaline DESs is lighter in color than that treated with acidic DESs, and scanning electron microscopy (SEM) indicated that DES-pretreated corn stover comprises looser and coarser fibers than untreated corn stover. Although acidic and alkaline DESs are both highly efficient pretreatment solvents, the intrinsic alkalinity of the latter renders it able to selectively remove lignin, cleave hemicellulose-lignin bonds, and increase the area of contact between the cellulose and biodegradation enzymes, greatly increasing the enzymolysis efficiency. The findings of this study are expected to serve as a theoretical reference for the design and selection of novel biomass pretreatment DESs.