Abstract: Waterborne polyurethane (PU) is a kind of environment-friendly coating material, widely used in leather, textile, construction coating and other fields. As a coating for leather and textile, the water vapor permeability (WVP) of polyurethane determines the wearing comfort of clothing. However, the water vapor permeability of conventional waterborne polyurethane is poor and needs to be modified to obtain coatings with excellent WVP. CaCl₂ and Heptafluorodecyl trimethoxysilane (FAS-17) were used to modify diatomite to prepare hydrophobic diatomite base materials. The effects of modification conditions on the structure and properties of diatomite were investigated. The modified diatomite with excellent performance was combined with PU emulsion and the WVP of composite film was studied. The results indicate that the diatomite modified with 30wt%CaCl₂ and 0.8wt%FAS-17 present the best comprehensive performance with increased specific surface area and pore structure. The moisture-regulating performance is improved and further enhanced by surface hydrophobic modification of FAS-17. After modified diatomite FAS-17-CaCl₂-D with best performance is combined with PU, the WVP of FAS-17-CaCl₂-D/PU composite film increase first then decrease with the increasing of FAS-17-CaCl₂-D dosage, and the hydrophobicity is improved. The composite PU film with 1% of FAS-17-CaCl₂-D shows the largest WVP, which increased by 16.3% compared with pure PU film. The SEM-EDS reveal that the the surface and cross section of FAS-17-CaCl₂-D/PU composite film appear the characteristic elements of FAS-17-CaCl₂-D such as Si, Ca and F. The pores appeared at the interface between PU and FAS-17-CaCl₂-D provided channels for the transfer of water vapor, resulting in improved WVP. The WVP enhanced PU in this work are expected to be applied in textile and leather coatings to improve the thermal comfort.