A process for separating an alkylation product, which is characterized in that a liquid phase alkylation product from an alkylation reaction unit is introduced into a first heat-exchanger after being pressurized with a pressure pump and heat-exchanged with a vapor phase stream from the column top of a high-pressure fractionating column, then introduced into a second heat-exchanger and further heated to 100°C-150°C, then introduced into the high-pressure fractionating column and subjected to fractionation under a condition of 2.0MPa-4.0MPa, the vapor phase stream from the column top of the high-pressure fractionating column is heat-exchanged with the liquid phase alkylation product to be separated, a liquid phase stream from the column bottom of the high-pressure fractionating column is introduced into a low-pressure fractionating column and subjected to fractionation under a condition of 0.2MPa-1.0MPa, a low-carbon alkane is obtained from the column top of the low-pressure fractionating column, wherein the low-carbon alkane is a C3-C5 hydrocarbon with isoalkane as the main component, wherein the content of isoalkanes is higher than 50%, based on the total weight of the low-carbon alkane, a liquid phase stream obtained from the column bottom of the low-pressure fractionating column is an alkylation oil product, wherein the alkylation oil product is a mixture having a distillation range of 25°C-220°C and is mainly composed of isoalkanes, which comprise greater than 80%, and has an alkene content of less than 2%, and an isooctane content of greater than 50%, the vapor phase stream from the high-pressure fractionating column, which is heat-exchanged in the first heat-exchanger, is wholly condensed as liquid phase, and the condensed liquid phase, in one part, returns as reflux to the column top of the high-pressure fractionating column, and in the other part, returns to the alkylation reaction unit, and the low-carbon alkane from the low-pressure fractionating column returns to the alkylation reaction unit. a liquid phase alkylation product from an alkylation reaction unit is introduced into a first heat-exchanger after being pressurized with a pressure pump and heat-exchanged with a vapor phase stream from the column top of a high-pressure fractionating column, then introduced into a second heat-exchanger and further heated to 100°C-150°C, then introduced into the high-pressure fractionating column and subjected to fractionation under a condition of 2.0MPa-4.0MPa, the vapor phase stream from the column top of the high-pressure fractionating column is heat-exchanged with the liquid phase alkylation product to be separated, a liquid phase stream from the column bottom of the high-pressure fractionating column is introduced into a low-pressure fractionating column and subjected to fractionation under a condition of 0.2MPa-1.0MPa, a low-carbon alkane is obtained from the column top of the low-pressure fractionating column, wherein the low-carbon alkane is a C3-C5 hydrocarbon with isoalkane as the main component, wherein the content of isoalkanes is higher than 50%, based on the total weight of the low-carbon alkane, a liquid phase stream obtained from the column bottom of the low-pressure fractionating column is an alkylation oil product, wherein the alkylation oil product is a mixture having a distillation range of 25°C-220°C and is mainly composed of isoalkanes, which comprise greater than 80%, and has an alkene content of less than 2%, and an isooctane content of greater than 50%, the vapor phase stream from the high-pressure fractionating column, which is heat-exchanged in the first heat-exchanger, is wholly condensed as liquid phase, and the condensed liquid phase, in one part, returns as reflux to the column top of the high-pressure fractionating column, and in the other part, returns to the alkylation reaction unit, and the low-carbon alkane from the low-pressure fractionating column returns to the alkylation reaction unit.