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Urea is easily dissolved in water and converted into gaseous ammonia or ammonium and bicarbonate ions under the catalysis of urease. The effects of total solids (10%, 30%, 50% and 70%) and ratios of urea to corn stover (1∶100, 1∶20, 1∶10, 1∶2 and 7∶10) on residual lignin content of corn stover were studied systematically at 75℃ to achieve the delignification kinetics of urea-pretreated corn stover, and the urea pretreatment characteristics of corn stover at different total solids were compared. The delignification kinetics characteristics of corn stover during urea pretreatment were fitted based on three first-order reactions. The first, second and third terms were based on the initial, bulk and residual phases of delignification, respectively. The results showed that the residual lignin content of urea-pretreated corn stover was decreased as the ratio of urea to corn stover was increased firstly, reached minimum and then increased as the ratio of urea to corn stover was increased when total solids was less than 50%. In comparison, the residual lignin content of urea-pretreated corn stover was decreased as the ratio of urea to corn stover was increased during pretreatment with total solids of 70%. The two optimal lignin removal rates of 71.05% and 68.69% were achieved at total solids of 10% and 70% with urea to corn stover ratios of 1∶20 and 7∶10, respectively. A good linear relationship between the residual lignin content of urea-pretreated corn stover and the solid recovery yield was obtained under different pretreatment conditions. The delignification selectivity of ureapretreated corn stover was stable at 0.32~0.48g/g during pretreatment process. The delignification kinetics of urea-pretreated corn stover conformed to the three first-order models of initial, bulk and residual phases. The maximum delignification fraction of the bulk-phase reached up to 0.71. Therefore, the aqueous or gaseous ammonia converted from urea at low or high total solids were equally favorable for the delignification of corn stover. The results provided good insights into the urea pretreatment of lignocellulosic biomass.