In this article, two kinds of seaweed biomass-based porous carbons were prepared by a green and efficient method of microwave activation/H2O2 synergistic modification from seaweed biomass resources. The Hg-0 capture performance of seaweed-based porous carbons was tested in a fixed-bed reactor system. The results show that microwave activation greatly increases the surface area of porous carbons. The porous carbons (S8W30 and E8W30) modified with 30% H2O2 exhibit excellent Hg-0 removal capacity at 120 degrees C. NO, O-2, and low concentrations of SO2 in flue gas have a positive effect on the removal of Hg-0. Water vapor and high concentrations of SO2 exhibit a negative effect on the process of Hg-0 capture. The oxygen-containing functional groups (C=O and C-OH/C-O-C groups) on the surface of the modified porous carbon, which are mainly produced by H2O2 modification, are considered to be the main active sites for Hg-0 capture. The X-ray photoelectron spectroscopy (XPS) analysis shows that the adsorbed Hg-0 can be oxidized to Hg2+. The modified porous carbons exhibit good development and application prospects compared with commercial activated carbon (AC) or other activated carbons.