Amine-functionalized adsorbents have attracted much attention in removing indoor CO2 to reduce the risk of human health. In the current work, polyethyleneimine (PEI)-modified silica monolith (PEIx-silica) was synthesized through a sol-gel method using PEI with molecular weights of 750,000, 25,000, and 10,000 as template agents. The N-2 adsorption result revealed that the PEIx-silica monolith possesses a mesoporous structure with 25-45 nm pore size assembled by silica particles (60-80 nm), which was decreased with the decrease in PEI molecular weight. The adsorption capacities of PEIx-silica for reducing CO2 concentration below 0.1% from 0.5% were investigated. The results indicated that CO2 adsorption capacity of PEIx-silica increased as molecular weight of PEI decreased. PEI10,000-silica achieved CO2 adsorption capacities of 58.5 and 100.4 mg g(-1) under dry and humid conditions and showed cyclic stability during a 10 times adsorption-desorption process. Therefore, PEIx-silica monolith provides a promising strategy for controlling indoor CO2.