The layered organic montmorillonite (OMMT) was first exfoliated into nanosheets by hydroxyl-terminated polybutadiene (HTPB). The OMMT nanosheets with HTPB were then assembled into environment-friendly waterborne polyurethane (OWPU) emulsion and film by in situ polymerization with monomers such as isophorone diisocyanate (IPDI) and dimethylolpropionic acid (DMPA) and so on. Small-angle XRD and TEM examinations indicated that the OMMT nanosheets had uniform contrast and good integrity. DLS, EDS, TGA, LOI, and CONE examinations suggested that the emulsion particle size, elastic modulus, thermal stability, droplet resistance and flame retardant properties of OWPU had been significantly improved. Particularly, the elastic modulus of OWPU nanofilm was increased by a maximum of 59.4% and a peak heat release rate of OWPU nanofilm was decreased by a maximum of 36.9 %. In addition, the surface morphology of the burning char residue showed that the tile-shaped OMMT interpenetrates to form a covering layer with a cluster structure.