Specific surface area activated carbon was prepared from pyrolysis semi-coke of marine garbage ulva by using KOH. The effect of activation process on adsorption performance of activated carbon was investigated by means of iodine adsorption and methylene blue adsorption. The iodine adsorption value and methylene blue adsorption value of activated carbon could be up to1824.19mg/g、914.98 mg/g when the semi-coke activated under the optimal conditions of mass ratio of KOH/semi-coke 3：1, activated temperature 800℃, activated time 45 min. The FTIR showed that the surface functional group of activated carbon contained a large number of hydroxyl groups. The SEM showed that the surface of activated carbon was very rough and had plenty of pore structure. BET surface area and Langmuir surface area were up to 2616.3 m2/g and 4883.5m2/g respectively. Activated carbon had micro- and mesostructure with pore size of 2.73 nm, so that it had the potential as the material of energy storage and environmental protection.