The provided is a process for separating by absorption the pyrolysis gas from preparation of lower carbon olefins, wherein a primary absorbent and a secondary absorbent are introduced into the demethanizer to separate by absorption the feedstock of the demethanizer through countercurrent contact therewith at a moderate temperature and pressure, thereby to obtain a top fraction primarily comprising hydrogen and methane and a bottom fraction primarily comprising the absorbents and C2+ fraction, wherein the primary absorbent essentially is a mixed Cn or Cn+ fraction, the secondary absorbent essentially is a Cn′ alkane fraction or mixed Cn′ or Cn′+ fraction, and wherein n and n′ are independently 3, 4 or 5 with the proviso when the secondary absorbent is a mixed fraction, n′ is not 3.