Abstract: Abstract: Urea is a common type of low-cost nitrogen fertilizer with a nitrogen content of 46%. Furthermore, urea is easily dissolved in water and then converted into gaseous ammonia or ammonium and bicarbonate ions upon catalysis by urease. To date, urea has served as an additive to pretreat the softwood spruce, hardwood birch, bamboo, and rice straw, because it can be transported, stored, and used without special equipment or conditions. However, the pretreatment of lignocellulosic biomass using only urea has not been extensively studied, especially for the pretreatment at high solids loading (TS). Furthermore, TS is a key factor in aqueous or gaseous ammonia production from urea. Therefore, this study aims to explore the effect of urea as a single reagent on the maize stover pretreated with TS. Meanwhile, the urease in soybean flour was used to promote the conversion of urea to ammonia, in order to realize the spontaneous pretreatment of maize stover with gaseous NH3 at relatively low temperature. A variable-controlling and full factorial experiment was carried out at a relatively high TS. A systematic investigation was made to explore the effects of TS (30%-70%), soybean flour to maize stover ratio (1:40-1:10), urea to maize stover ratio (1:10-1:1), pretreatment temperature (60-80℃), and pretreatment time (2-14 d) on the compositional characteristics, the surface morphology, and group structure of maize stover. The results indicated that the lignin content of pretreated maize stover increased, whereas, the content of glucan, xylan and araban increased firstly and then decreased, as the TS increased. The carbohydrates in the maize stover were well protected after the urea pretreatment with 50% TS. The urease in soybean flour increased the glucan and araban content of urea-pretreated maize stover, but no influence on the lignin and xylan content. It was found that the 1:20 ratio of soybean flour to maize stover was favorable for the urea pretreatment of maize stover. The lignin and xylan content of pretreated maize stover decreased slightly, with the increase of urea to maize stover ratio and pretreatment time, but differed insignificantly as pretreatment temperature. Nevertheless, the glucan content of pretreated maize stover increased significantly, with the increase of urea to maize stover ratio, pretreatment temperature, and time. The araban content of pretreated maize stover decreased slightly with the increase of pretreatment time, but remained unchanged with the increase in the urea to maize stover ratio and pretreatment temperature. The solid recoveries of maize stover were 63.69%, 69.73%, and 73.01% at three temperatures (80, 70, 60℃) for 10, 12, and 14 d, after pretreatment with fixed 50% TS, 1:20 ratio of soybean flour to maize stover, and 1:1 ratio of urea to maize stover. Under the same conditions, the recoveries were 97.24%, 93.75% and 92.34% for glucan, 61.63%, 66.70% and 68.09% for xylan, and 64.55%, 66.92% and 64.24% for araban. The contents reached 14.20%, 15.91% and 17.34% for lignin, 51.75%, 45.57% and 42.87% for glucan, 20.67%, 20.40% and 19.92% for xylan, and 4.23%, 4.67% and 4.28% for araban. The C-O group (1 032 cm-1) related with cellulose, hemicellulose and lignin of maize stover fluctuated significantly, and the lignin and hydroxycinnamate linkages cleaved obviously during urea pretreatment. Consequently, it is feasible for the maize stover pretreatment with spontaneous gaseous NH3 from urea at higher TS. The finding can provide a potential research basis for urea pretreatment.