MgAl layered double hydroxides (LDHs) containing tripolyphosphate (P3O105-) (P-LDHs) were prepared by the anion exchange method; subsequently, aminopropyltriethoxysilane (APTS) was grafted onto the surface of P-LDHs to prepare S-MgAl LDHs (S-LDHs) through induced hydrolysis silylation. The dispersion, flame retardancy, and smoke suppression of N-LDHs (MgAI LDHs with nitrate in its interlayer), P-LDHs, and S-LDHs in polyurethane elastomer (PUE) were investigated. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that S-LDHs could disperse uniformly in PUE. The cone calorimetry and smoke density test indicated that S-LDHs performed best in flame retardancy and smoke suppression. By analyzing the char layer with Laser Raman spectroscopy (LRS), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), it could be concluded that, besides the catalytic charring effect of LDHs, the stable structure of -P(=O)-O-C- and-P(=O)-O-Si- could improve the thermal oxidative resistance and stabilize the char layer, resulting in better flame retardancy and smoke suppression.