A series of thermoplastic polyurethanes (TPUs) with the same molecular weight but different soft/hard segment ratios were synthesized by in-situ condensation polymerization using poly (tetramethylene glycol) (PTMG) as the polyol and methylene diphenyl diisocyanate (MDI) as the isocyanate. The weight fractions of the hard segments were varied from 0.0 to 0.4. The structures of the TPU series were analyzed by Fourier transform infrared spectroscopy and gel permeation chromatography. The changes in the thermal and optical properties due to the hard segment crystallinity were also measured by differential scanning calorimetry and UV-Vis spectroscopy. Increasing the hard content promoted phase separation and served as absorption blocks in the TPU. The water vapor permeability of the TPU films with different soft/hard ratios ranged from 223.63 to 116.26 g/㎡·day.