A method for manufacturing a crosslinked polyolefin separator, including the steps of: (S1) introducing polyolefin having a weight average molecular weight of 200,000-1,000,000, a first diluting agent, alkoxysilane containing a carbon-carbon double bonded group, an initiator and a crosslinking catalyst to an extruder; (S2) introducing a second diluting agent to the extruder, followed by mixing, to carry out reactive extrusion of a silane-grafted polyolefin composition; (S3) molding and orienting the reactive extruded silane-grafted polyolefin composition in a form of a sheet; (S4) extracting the first diluting agent and the second diluting agent from the oriented sheet to obtain a silane-grafted polyolefin porous membrane; (S5) thermally fixing the porous membrane; and (S6) crosslinking the porous membrane in the presence of water and a crosslinked polyolefin separator are provided. According to the method, it is possible to provide a separator which shows a decrease in intensive silane crosslinking on the surface thereof.