The invention discloses a system and a method for recovering and utilizing pressure energy of tail gas from an olefin separation device. The system comprises a precooler, a gas-liquid separator and anexpander. The system is simple and mature, has less investment, and recovers ethylene and propylene in the tail gas by utilizing the pressure energy of the tail gas. The process provided by the invention guarantees less loss of tower-top ethylene and propylene of a demethanizing tower, so a tower-top liquid of a low-pressure depropanizing tower and a tower-bottom liquid of a dethanizing tower canbe selected to replace propane or a carbon-four washing liquor in the prior art; thus, problems like large equipment and high operation cost due to long circulation process in the prior art are avoided; and a newly-built device provided by the invention can reduce equipment investment and installation cost.