The utilization of landfill gas from municipal waste landfills has become a hot research topic. The separation and purification of carbon dioxide (CO2) and methane (CH4) from landfill gas has attracted widespread attention. In this study, ethanolamine (MEA), diethanolamine (DEA), and diethylenetriamine (DETA) were used to modify silica gel to improve the adsorption separation performance of silica gel for CO2/CH4. The results showed that the improvement in CO2/CH4 separation performance by different organic amines followed the order of MEA > DEA > DETA. That is, the most effective modification was achieved by MEA. The adsorption of CO2 onto MEA-modified silica gel was 84.83% higher than that on unmodified silica gel. The effect of MEA concentration on the adsorption separation of CO2/CH4 was also studied. When the ratio of MEA to methanol was 1:6, the saturated CO2 adsorption capacity of the modified silica gel reached 3.90 mmol/g, which was 84.83% higher than that of the unmodified silica gel. These conditions yielded the best CO2/CH4 separation performance. The corresponding separation ratio was 8.51, which was 6.25% higher than that of the unmodified silica gel under the same conditions.