A coupling polymer, abbreviated SPEO-MDI-SPEO (MSPEO), was synthesized by a simple reaction between 4,4'-methylenediphenyl diisocyanate (MDI) and stearylpoly(ethylene oxide) (SPEO, M-n 1900), and the blended poly(ether urethane) (PEU) films of PEU-MSPEO were prepared by a solution process. According to the analysis of ATR-FTIR, it was proved that when MSPEO was blended into the PEU matrix, the middle blocks (M-block) of MSPEO could incorporate with the hard blocks of PEU chains through the linkage of a H bond, leading to the improvement in the blending stability. The surface modification was finally accomplished by the self-motion of MSPEO since the elastomeric property of the matrix permits the modifiers to move freely. In the case of air, due to the relatively poor compatibility and rather low surface energy of the stearyl end groups, they migrated to the polymer-air interface, and thus the connected PEO chains were also enriched there by the tow of the end groups. However, in the case of water, the hydrophilic PEO chains migrated to the outermost layer of the surface, and finally they were enriched on the polymer-water interface, while the hydrophobic stearyl end groups would bend down back into the surface. Therefore, a PEO chain-loop structure was finally formed. Furthermore, also based on the mechanism of self-motioned surface modification, even if a part of MSPEO on the surface would have been washed away by water, the continuous makeup from the bulk would still be able to maintain the quantity of the PEO chains on the interface for a long time. All the results were Verified by H-1 NMR, ATR-FTIR, XPS, and contact angle measurements.