The utility model relates to a Y-shaped rotor polymerization reactor comprising a horizontal barrel and two main shafts, wherein the cross section of an inner cavity of the barrel is in a double-circular shape; the two main shafts are respectively located at two circle centers of the double circles; the main shafts are provided with a plurality of Y-shaped rotors; each Y-shaped rotor comprises a disc body and a hub arranged in the middle of the disc body; the disc body is composed of three spokes and three scrapers; end covers at two ends of the barrel are both provided with two bearing devices for mounting the two main shafts, respectively; each bearing device comprises a bearing and a shaft seal located at the inner side of the bearing; each shaft seal comprises a shaft sleeve and a plurality of shaft seal sealing rings embedded in the shaft sleeve. During the normal working process, the Y-shaped rotor polymerization reactor is capable of automatically cleaning retention layers on the inner side of the barrel and the surfaces of the main shafts, good in sealing effect at shaft ends, easy and reliable to assemble, low in failure rate, long in service life and mainly used for polymerization reaction of lactic acid.