A novel adsorbent of mesoporous carbon with high specific surface area was successfully prepared by hydrothermal and template method, using sodium lignosulfonate (LSs) as a raw material and cetyltrimethylamrnonium bromide (CTAB) as a template agent. The mesoporous carbon was characterized by SEM, TEM, BET, FTIR, and XPS. The formation mechanism of the mesoporous carbon was analyzed. The adsorption of uranium(VI) on the mesoporous carbon from the simulated aqueous solution and actual radioactive wastewater was respectively investigated. The optimum conditions for U(VI) adsorption were determined by studying experimental variables including pH, contact time, sorbent dose, initial concentration, and temperature. The results indicated that the maximum adsorption capacity of the mesoporous carbon for U(VI) in the simulated aqueous solution and actual radioactive wastewater was respectively 109.46 mg/g at pH 5.5 and 328.15 K and 195.6 mg/g at pH 5.5 and the initial U(VI) concentration of 189.75 mg/L. The adsorption data could be well described by the pseudo-second-order model and Freundlich isotherm model. The adsorption of U(VI) on the mesoporous carbon was an endothermic and spontaneous process. The adsorption mechanism may be a complex chemical reaction between uranium and the oxygen-containing functional groups on the mesoporous carbon.