Magnesium Hydroxide (MH) is a widely used, commercialized and low cost flame retardant. However, as inorganic material its poor compatibility with polymer materials and low efficiency has limited its application, especially in some brittle matrix such as PLA. Therefore, a surface modification that can enhance the interface between MH and polymer is necessary. In this study, a kind of novel modifier (PPD-PTES) for MH was successfully synthesized from (3-Aminopropyl) triethoxysilane and phenylphosphonic dichloride aiming for improving not only the compatibility of MH with polymer but also its efficiency. Fourier transform infrared spectroscopy (FTIR), 1H, 31P-nuclear magnetic resonance (NMR) were carried out to characterize its structure. Then MH was modified using PPD-PTES by wet method and the resulting m-MH was characterized by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and Transmission electron microscopy (SEM) and added into PLA. The flame retardant, thermal degradation and mechanical properties of the PLA/MH composites were investigated by vertical burning (UL94), limited oxygen index (LOI), microscale combustion calorimetry (MCC), thermogravimetric analysis (TGA) and tensile test.