The practical applications of lithium-selenium (Li-Se) batteries are impeded due to the low utilization of active selenium, sluggish kinetics, and volume change. The development of highly efficient host materials to suppress high-order polyselenide shuttling and accelerate Li2Se conversion is essential for Li-Se batteries. Herein, a theoretical design of a Co@C2N monolayer as a host material for ultra-high areal capacity Li-Se batteries is proposed by first-principles calculations. The investigations of the lowest energy configurations, binding energies, and the charge transfer indicate that the Co@C2N monolayer could alleviate the reciprocating motion of high-order polyselenides and improve the cycling performance. Further electronic property calculations show that the semi-metallic characteristics of the Co@C2N monolayer material are retained even after chemical adsorption with Se8 or Li2Sen molecules, which is beneficial for the utilization of active selenium. In addition, the crucial catalytic role of the Co@C2N monolayer is investigated and the results indicate that the Co@C2N monolayer could facilitate the formation and decomposition of Li2Se molecules during the discharge and charge processes. Our present work would not only provide a deep understanding on the anchoring and catalytic effect of the Co@C2N monolayer, but also demonstrate a general principle for the rational design and screening of advanced materials for high energy density Li-Se batteries.