The invention discloses a preparation method of a porous polymer in-situ microfiber oil-water separation material. The preparation method comprises the following steps: (1) carrying out melt blendingon a polymer a and a pore-forming polymer b to obtain a microfiber phase, namely a product A; (2) performing in-situ microfibrillation on the product A and a thermoplastic elastomer polymer c througha micro-nano laminated co-extrusion device to obtain an elastomer-based in-situ microfiber composite material, namely a product B; (3) etching the thermoplastic elastomer matrix in the product B by using a good solvent of the thermoplastic elastomer to obtain polymer a/pore-forming polymer b blend microfibers, namely a product C; and (4) etching the pore-forming polymer b in the product C by usinga good solvent of the pore-forming polymer to obtain the porous polymer an in-situ microfiber oil-water separation material. The porous polymer in-situ microfiber oil-water separation material prepared by the invention has super-hydrophobic and super-oleophylic properties, can effectively collect oil stains to realize oil-water separation, and has high oil absorption capacity. And the preparationprocess is simple, and industrial production can be realized.