Magnetic adsorbent shows good development prospects for separation of He from flue gas because it can be recycled. In this work, a novel magnetic biomass porous carbon adsorbent was developed by loading active ingredients (Ce and Fe mixed oxides) on renewable maize straw carbon with large specific surface area. Microwave activation and ultrasound treatment were applied to improve porous structure of maize straw carbon and distribution of active components. The influence of process parameters on Hg-0 capture and the removal mechanism were also investigated. The results reveal that CeFe11%(3/5)/MSWU700 possesses the optimal Hg-0 removing performance and adsorption capacity at 140 degrees C. The characterization results show that microwave activation can greatly increase the specific surface area of biomass carbon to form excellent porous structure, and ultrasound-assisted impregnation can facilitate the dispersion of active ingredients on the surface of adsorbent. The presence of Hg2+ on the surface of CeFe11%(3/5)/MSWU700 implies that chemisorption occurred during the Hg-0 removal process, which is also demonstrated by the well-matched pseudo-second-order reaction model. In the removal process of He, the highly active chemisorbed oxygen was largely consumed, and the conversion of Ce4+/Fe4+ to Ce3+/Fe2+ was found. The magnetic adsorbent CeFe11%(3/5)/MSWU700 shows a large Hg-0 adsorption capacity (it is up to 7230.8 mu g/g, which is far more than the common activated carbons and magnetic adsorbents), showing excellent application prospect.