This research demonstrates the production of mesoporous activated carbon from sargassum fusiforme via physical activation with carbon dioxide. Central composite design was applied to conduct the experiments at different levels by altering three operating parameters. Activation temperature (766–934 ℃), CO2 flow rate (0.8–2.8 L?min?1) and activation time (5–55 min) were the variables examined in this study. The effect of parameters on the specific surface area, total pore volume and b-out rate of activated carbon was studied, and the influential parameters of methylene blue adsorption value were identified employing analysis of variance. The optimum conditions for maximum methylene blue adsorption value were: activation temperature=900 ℃, activation time=29.05 min and CO2 flow rate=1.8 L?min?1. The activated carbon produced under optimum conditions was characterized by BET, FTIR and SEM. The adsorption behavior on congo red was studied. The effect of parameters on the adsorbent dosage, temperature, PH and initial congo red concentration was investigated. The adsorption properties of the activated carbon were investigated by kinetics. The equilibrium removal rate and maximum adsorption capacity reaches up to 94.72％, 234 mg?g?1, respectively when initial congo red concentration is 200 mg?L?1 under adsorbent dosage (0.8 g?L?1), temperature (30 ℃), PH7.