The invention relates to a method for pretreating agricultural waste biomass by adopting a sodium hydroxide/urea/water system, and belongs to the fields of development and utilization of an agricultural waste resource utilization technology and a new energy technology. The method comprises the following steps: (1) mechanical treatment of an agricultural waste lignocellulose material, namely carrying out mechanical treatment to reduce the size of a lignocellulose substrate and increase the specific surface area of the substrate; (2) chemical treatment of the agricultural waste lignocellulose material, namely adding the lignocellulose substrate with reduced size to the sodium hydroxide/urea/water system, filtering the obtained mixture and washing filter residues to be neutral; and (3) agricultural waste enzymatic saccharification. Treatment of a lignocellulose system with the sodium hydroxide/urea/water system built by the method has significant advantages of simplicity in operation, low cost, short consumed time and high enzymatic saccharification efficiency, and can be put into large-scale production; no fermentation inhibitor is generated; chemical medicines such as sodium hydroxide and urea adopted by the method disclosed by the invention are low in price; and liquid waste is easy to recycle.