Carbon xerogel is a nano-scale adsorption material with abundant mesopores and large adsorption capacity. A series of mesoporous carbon xerogels (MCX) were synthesized in the process of polycondensation of resorcinol and formaldehyde using different resorcinol/sodium carbonate catalyst molar ratios (R/C = 100~3000). And the adsorption behavior of the MCX for the azo dye reactive blue 13 (RB13) from dyeing wastewater was investigated in this study. Results showed that variations in both specific surface area and pore size distribution of MCX were closely related to the amount of catalyst. The MCX exhibited maximal surface area, abundant mesoporosity, and highest adsorption capacity for RB13 when the molar ratio of R/C was 1000. The adsorption of RB13 by MCX1000 proceeded rapidly in the first 50 min and reached equilibrium in 180 min. The adsorption process was exothermic, spontaneous, and can be well fitted by the pseudo-second-order and Temkin models, with the maximum sorption capacity of 184.36 mg·g^-1. The adsorption of RB13 onto MCX was attributed to physicochemical adsorption. The main adsorption mechanisms of the MCX for RB13 were the electrostatic interaction, hydrogen bonding and van der Waals forces between dye molecules and functional groups, such as -OH, and -C-O, etc.