A method for manufacturing a crosslinked polyolefin separator and the crosslinked polyolefin separator obtained by the method are provided. The method includes (S1) mixing polyolefin, a diluting agent, an initiator and alkoxysilane containing a carbon-carbon double bonded group to an extruder, and then carrying out extrusion to obtain a silane-grafted polyolefin composition; (S2) molding and orienting the extruded silane-grafted polyolefin composition in the form of a sheet; (S3) introducing the oriented sheet to an extraction water bath containing a crosslinking catalyst to extract the diluting agent and perform aqueous crosslinking; and (S4) thermally fixing a resultant aqueous crosslinked product. The method can provide a separator having a high meltdown temperature and improved heat shrinkage.