In this study, a type of carbon aerogel was synthesized by main-chain benzoxazine using a quasi-solvent-free method. The benzoxazine used 4,4′-thiobisphenol (TBP) and bisphenol A as two different phenol sources for benzoxazine and used formaldehyde and triethylenetetramine as synthetic raw materials. Two different solvents, dioxane and dimethylformamide, were selected in the synthesis of carbon aerogel, and the influence of the solvent on thermodynamics and molecular structure was studied. Sulfur atoms were added to the aerogel by using TBP as a phenol source, and the effects of sulfur on structure, curing process, and thermal properties on main-chain benzoxazine aerogel were studied by comparing the products synthesized from different phenol source materials. The precursor was carbonized to obtain the carbon aerogel. Analyses using a scanning electron microscope, an elemental analyzer, and a gas adsorption analyzer provided information about the microstructure, element content, and carbon dioxide (CO 2 ) adsorption properties of sulfur-containing and nonsulfur-containing carbon aerogels.