Capture of CO2 and utilization of marine biomass resources have attracted more and more attention due to climate change and lack of terrestrial resources. In this article, using sargassum and enteromorpha as raw materials, two kinds of seaweed porous carbons adsorbents were prepared using urea doping and KOH activation to capture CO2 from flue gas. Characterization methods such as BET, pH, SEM, FTIR, and XPS were used to investigate the physicochemical properties of seaweed porous carbon adsorbents. The main parameters and flue gas components that may affect the CO2 adsorption, as well as the adsorption mechanism were also studied. The results show that compared with inactivated biochars, all of the specific surface areas, the surface functional groups, and the CO2 adsorption capacities of the seaweed porous carbons adsorbents (e.g., NSK (2) and NEK (2)) are significantly increased. The highest CO2 adsorption capacities of NSK (2) and NEK (2) are up to 4.06 and 3.25 mmol/g at 25 degrees C, respectively. Moreover, NSK (2) and NEK (2) still show excellent CO2 adsorption performance after 10 regeneration experiments (reaching 3.69 and 2.93 mmol/g, respectively). Other flue gas components such as O-2 and NO have no effect on the CO2 adsorption performance of NSK (2) and NEK (2), while H2O (g) and SO2 exhibit an inhibitory effect. The pseudo-first-order model can reasonably explain the CO2 adsorption processes of two kinds of seaweed porous carbons adsorbents, showing that external mass transfer is the main control step for the CO2 capture. Thermodynamic analysis results demonstrate that physical adsorption is dominant in the CO2 adsorption process over NSK (2) and NEK (2), and chemical adsorption only plays a supplementary role.