The invention provides a gas-liquid separation system for a high-gas-oil-ratio oil well. The gas-liquid separation system comprises a casing arranged in the well, as well as a double-passage gassy liquid separator, a gassy liquid delivery passage assembly and a liquid inlet device assembly which are arranged in the casing and sequentially connected from top to bottom through tubing couplings, wherein the length of the casing is equal to the distance between a wellhead and a well bottom; the top of the double-passage gassy liquid separator is connected with an oil pump, and the bottom of the liquid inlet device assembly is connected with a sand settling pipe. According to the system, the rotational flow velocity and the rotational flow radius are increased, and gas flow passages are additionally arranged, so that the underground gas, sand and oil separation efficiency of a high-gas-oil-ratio mechanical oil well is improved; in addition, a gassy liquid passage check valve and a double-layer telescopic pipe are arranged, so that the function of preventing a flushing fluid from flowing backward during flushing is realized, and the aim of conveniently dismounting an upper liquid column of a packer during underground operation can be also fulfilled.