The invention discloses a method for preparing ultra-high molecular weight polyethylene fiber. The method comprises the following steps: 1) heating and mixing poly-alpha-olefin oil and ultra-high molecular weight polyethylene of which the molecular weight is greater than 1 million, and pre-swelling and dissolving the mixture to form a semi-dilute solution or a non-solution mixture; 2) conveying the mixture into a screw extruder through a gear delivery pump, extruding by adopting a single-screw or double-screw or three-screw extruder to obtain an extrusion material; 3) conveying the obtained extrusion material to a spinneret plate by using a melt booster through a filter, spraying the extrusion material from holes of the spinneret plate, and cooling; 4) drawing a plurality of cooled filaments by adopting a roller multistage traction device, performing multiplication and multistage drawing to obtain the ultra-high molecular weight polyethylene fiber.