A novel adsorbent of CO2 from biogas was prepared by synthesizing and modifying the mesoporous molecular silica of SBA-15 with methyl-diethyl-amine (MDEA) and piperazine (PZ). The adsorbent showed good performance in separating CO2 from biogas. The loaded amines did not change the ordered structure of SBA-15, but enhanced its adsorption of CO2. The adsorbents were characterized by X-ray powder diffraction (XRD) and N2 adsorption/desorption. With the increase in MDEA loading, the surface area, pore size, and pore volume of the MDEA-loaded SBA-15 decreased. The modification of amines enlarged the difference between the equilibrium adsorption of CO2 and CH4. Quantitatively evaluated on the basis of the breakthrough curves, the separation factors between CO2 and CH4, was increased more than seven fold due to the MDEA modification. With mixed-amine (MDEA+PZ) modification, the separation factors between CO2 and CH4 was further improved. In addition, not only the adsorbent was regenerable by purging with the purified gas, but also the adsorption performance is stable in adsorption cycles. Effect of moisture on adsorption of CO2 is investigated and the results show the increase in the adsorption performance.