This article examines the effects of the soft-segment composition on the structure and physical properties of waterborne polyurethane (WBPU) based on polycaprolactone-poly(ethylene glycol)-polycaprolactone (PCL-PEG-PCL) triblock copolydiol as the soft segment. The molecular weight of PEG in the soft-segment composition and soft-segment content(SSC) are varied in this study. The water-vapor permeability (WVP) for the WBPU-coated nylon fabric is also studied. The results showed that the glass transition temperatures (T-g’s) of the soft segment decreased and its temperature range (Delta T-g’s) narrowed with increase of SSC up to 63 wt % and decrease of the PEG molecular weight. The dynamic mechanical analysis results showed that the alpha-peak height of the soft segment increased with SSC when the SSC was less than 63 wt %. However, when SSC was more than 63 wt %, the alpha-peak height became smaller with increasing SSC due to the crystallization of the soft segment. At the same SSC, the number of spherulites was reduced and the spherulites become larger with decrease of the PEG molecular weight. As for the mechanical properties of the WBPU cast film, the breaking stress decreased and the breaking elongation increased with increasing SSC or decreasing PEG molecular weight. For the WBPU-coated nylon fabrics, either higher SSC or higher PEG molecular weight improves the WVP.