Bioethanol produced from lignocellulose is a renewable energy substitute for traditional fossil fuels. Poplar wood as forest waste is popular in bioethanol production. Nonetheless, the complex structure of lignocellulose leads to low reducing sugar and ethanol yields. Thus, lignocellulose pretreatment is necessary to promote enzymatic hydrolysis. Deep eutectic solvents (DESs) have good dissolution capacity, low vapor pressure, a simple synthesis procedure, low synthesis cost and low toxicity. More and more researchers have begun paying attention to the application of DESs in lignocellulose pretreatment. In this work, poplar wood was pretreated using a series of basic DESs based on diol. The effects of the DES species, the basicity of the solvents, the pretreatment temperature and the pretreatment time on the effectiveness of pretreatment and enzyme hydrolysis for poplar wood were investigated, and characterization analysis (Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy) of poplar wood was carried out to reveal the pretreatment mechanism. The best pretreatment effect was obtained from K: 1, 2-PG, which removed 89.2% and 71.6% of the lignin and hemicellulose, respectively, while preserving 97.5% of the cellulose at 130 °C for 7 h. This enhanced the reducing sugar yield to 82.5% relative to that of the raw sample (3.3%) after 72 h of hydrolysis. The results of the characterization analysis demonstrated that lignin and hemicellulose were removed. Therefore, the DES based on K: 1, 2-PG is a promising solvent for poplar wood pretreatment, and could improve the industrial production of reducing sugar and bioethanol.