NOVELTY - Compressed and dried low carbon olefin mixed gas and liquid are introduced into a high pressure depropanizer, 4C rich in 3C collected from bottom of the high pressure depropanizer is supplied to a low pressure depropanizer, and the liquid at the top of the low pressure depropanizer is pumped into the high pressure depropanizer. Gas collected from the upper portion of high pressure depropanizer is introduced into a compressor. The gas after pressure lifting through cooling is introduced into the demethanizer, and the gas at top of demethanizer is supplied into bottom of an absorption tower.    USE - Method for separating low carbon olefin mixed gas.    ADVANTAGE - The low carbon olefin mixed gas is economically separated.    DETAILED DESCRIPTION - Compressed and dried low carbon olefin mixed gas and liquid are introduced into a high pressure depropanizer, 4C rich in 3C collected from bottom of the high pressure depropanizer is supplied to a low pressure depropanizer, and the liquid at the top of the low pressure depropanizer is pumped into the high pressure depropanizer. Gas collected from the upper portion of high pressure depropanizer is introduced into a compressor. The gas after pressure lifting through cooling is introduced into the demethanizer, and the gas at the top of the demethanizer is supplied into the bottom of an absorption tower. Absorbent from the upper portion of the absorption tower is introduced, mass transfer is carried out, 1C which is not absorbed by the absorbent and lighter component are taken from the top of the absorption tower. The raw absorbent is supplied to a regenerating column, and 2C is separated from the absorbent in the regenerating column. The absorbent is recycled after heat recovery. The absorbent is chosen from alcohol containing oxygen atoms and 1-4 carbon atoms in the molecule, ethers or hydrocarbons with 3-5 carbon atoms and does not containing oxygen atom. 2C-3C obtained at the bottom of the demethanizer is supplied to a deethanizer. 2C is separated from 3C in the deethanizer, 3C is supplied into a propylene tower, propylene product is obtained at the top of the propylene tower, and propane by-product is obtained at the bottom of the tower. 2C product obtained at the top of deethanizer is supplied to the ethylene tower, large amount of ethylene is obtained at the top of ethylene tower, and the ethylene and the 2C gas obtained at the top of the regenerating column are supplied to reaction system after heat recovery. Ethane and a small amount of ethane obtained at the bottom of the ethylene tower are supplied to an ethane tower, polymerized ethylene is obtained at the top of the ethane tower, and ethane is obtained at the bottom of the tower. Ethane is supplied together with gas at the top of absorption tower as fuel gas after cold energy recovery. 4C obtained at the bottom of the low pressure depropanizer is divided into two portions, one portion is supplied to 4C tower, and 4C is separated from 5C in 4C tower. 4C and propane as liquefied gas are supplied to tank region. 5C as gasoline product is supplied to tank region, and other portion is returned.