NOVELTY - Preparing biological natural gas by biogas comprises compressing the biogas to 0.35-0.7 MPa and letting it enter to the absorption tower; contacting the biogas flowing from the bottom up with the absorbent-ethanol amine solution flowing from the top down reversely; absorbing the carbon dioxide and hydrogen sulfide; obtaining the purified biological natural gas from the top of the absorption tower; and heating the rich liquid from the absorption tower at 115-125 degrees C by the heater to decompose carbon dioxide and hydrogen sulfide in the deabsorption tower.    USE - The process is useful for preparing biological natural gas by biogas (claimed).    ADVANTAGE - The process makes use of excellent absorbent ethanolamine with big dissolving degree to absorb carbon dioxide and sulfide in the biogas; thus, the carbon dioxide and sulfide are removed; and the content of the methanol, also called biological natural gas, is improved to more than 95%, which satisfies the demand of the natural gas component. Moreover, the biogas application is expanded; the use value of the biogas is improved; and it has notable economic, social and environment benefits.    DETAILED DESCRIPTION - Preparing biological natural gas by biogas comprises compressing the biogas to 0.35-0.7 MPa and letting it enter to the absorption tower; contacting the biogas flowing from the bottom up with the absorbent ethanolamine solution flowing from the top down reversely; absorbing the carbon dioxide (CO2) and hydrogen sulfide (H2S); obtaining the purified biological natural gas from the top of the absorption tower; and heating the rich liquid from the absorption tower at 115-125 degrees C by the heater to decompose CO2 and H2S in the deabsorption tower; where the pressure of the absorption tower is 0.35-0.7 MPa, and the temperature is normal temperature; and the temperature of the deabsorption tower is 105-110 degrees C.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram of the process for preparing biological natural gas by biogas (Drawing includes non-English language text).